MAINT: Drop Index.sym_diff
Deprecated in 0.18.1

xref #12591, #12594

(a nice deprecation removal unlike `pd.eval` :smile:)

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=h1) Report
> Merging [#16760](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/18f7b1ccee1c723cac7f23a16099d08c17ed0a91?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16760/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16760      +/-   ##
==========================================
+ Coverage   90.93%   90.93%   +<.01%     
==========================================
  Files         161      161              
  Lines       49282    49281       -1     
==========================================
  Hits        44815    44815              
+ Misses       4467     4466       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.7% <100%> (ø)` | :arrow_up: |
| #single | `40.17% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `95.81% <100%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.33% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16760?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/16760?src=pr&el=footer). Last update [18f7b1c...4dd740a](https://codecov.io/gh/pandas-dev/pandas/pull/16760?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/16760?src=pr&el=h1) Report
> Merging [#16760](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/18f7b1ccee1c723cac7f23a16099d08c17ed0a91?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16760/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16760      +/-   ##
==========================================
+ Coverage   90.93%   90.93%   +<.01%     
==========================================
  Files         161      161              
  Lines       49282    49281       -1     
==========================================
  Hits        44815    44815              
+ Misses       4467     4466       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.7% <100%> (ø)` | :arrow_up: |
| #single | `40.17% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `95.81% <100%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.33% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16760?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/16760?src=pr&el=footer). Last update [18f7b1c...4dd740a](https://codecov.io/gh/pandas-dev/pandas/pull/16760?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/16760?src=pr&el=h1) Report
> Merging [#16760](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1265c27f4bbd06e1bb75f846139a164bdadd5b31?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/16760/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16760      +/-   ##
==========================================
- Coverage   90.93%   90.93%   -0.01%     
==========================================
  Files         161      161              
  Lines       49280    49279       -1     
==========================================
- Hits        44814    44813       -1     
  Misses       4466     4466
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.7% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.17% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `95.81% <100%> (-0.01%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16760?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/16760?src=pr&el=footer). Last update [1265c27...4bfb2b7](https://codecov.io/gh/pandas-dev/pandas/pull/16760?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jorisvandenbossche : Can this be merged? Yes, thanks!