[PERF] Get rid of MultiIndex conversion in IntervalIndex.is_unique 
-  xref #24813 
-  benchmark added
-  passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
-  whatsnew entry

This PR follows #25159 (in which I broke something when merging).
```
All benchmarks:

       before           after         ratio
     [304d8d4b]       [4ec1fe97]
     <master>         <intv-is-unique>
        228±0.2ns        230±0.2ns     1.01  index_object.IntervalIndexMethod.time_is_unique(1000)
-         450±7ns          277±2ns     0.62  index_object.IntervalIndexMethod.time_is_unique(100000)
```

The approach this time doesn't worsen the performance.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=h1) Report
> Merging [#26391](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/612c24433b8486eb6b6b0013354dd6d6a296c100?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `93.33%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26391/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26391      +/-   ##
==========================================
- Coverage   91.69%   91.68%   -0.01%     
==========================================
  Files         174      174              
  Lines       50749    50763      +14     
==========================================
+ Hits        46534    46543       +9     
- Misses       4215     4220       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.19% <93.33%> (ø)` | :arrow_up: |
| #single | `41.15% <0%> (-0.17%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `95.17% <93.33%> (-0.06%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=footer). Last update [612c244...d11acd6](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=h1) Report
> Merging [#26391](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/612c24433b8486eb6b6b0013354dd6d6a296c100?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26391/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26391      +/-   ##
==========================================
- Coverage   91.69%   91.69%   -0.01%     
==========================================
  Files         174      174              
  Lines       50749    50754       +5     
==========================================
+ Hits        46534    46537       +3     
- Misses       4215     4217       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.2% <100%> (ø)` | :arrow_up: |
| #single | `41.15% <0%> (-0.17%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `95.33% <100%> (+0.1%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `93.54% <0%> (-0.4%)` | :arrow_down: |
| [pandas/plotting/\_style.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19zdHlsZS5weQ==) | `76.92% <0%> (-0.26%)` | :arrow_down: |
| [pandas/plotting/\_misc.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19taXNjLnB5) | `38.23% <0%> (-0.23%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.6% <0%> (-0.11%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.66% <0%> (-0.06%)` | :arrow_down: |
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `88.96% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `96.65% <0%> (-0.02%)` | :arrow_down: |
| ... and [8 more](https://codecov.io/gh/pandas-dev/pandas/pull/26391/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=footer). Last update [612c244...d3af9c9](https://codecov.io/gh/pandas-dev/pandas/pull/26391?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. @jschendel  @jreback Would you tell me if you have anything to implement? lgtm. @jschendel merge if you are ok with this. thanks @makbigc