REF: remove unreachable code in core.internals
@TomAugspurger it looks like _sparse_blockify is never hit.  Is that no longer reachable?
SparseBlock is gone, so `sparse_blockify` is indeed likely dead. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26800?src=pr&el=h1) Report
> Merging [#26800](https://codecov.io/gh/pandas-dev/pandas/pull/26800?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/129aaea46db94d5f8296bd5daeff9dc124b1a99e?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26800/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26800?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26800      +/-   ##
==========================================
+ Coverage   91.73%   91.76%   +0.02%     
==========================================
  Files         178      178              
  Lines       50774    50751      -23     
==========================================
- Hits        46578    46570       -8     
+ Misses       4196     4181      -15
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.35% <100%> (+0.03%)` | :arrow_up: |
| #single | `41.22% <60%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26800?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/26800/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `95.21% <ø> (+1.34%)` | :arrow_up: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/26800/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `94.38% <100%> (+0.25%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26800/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/26800/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.88% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26800/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.94% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26800?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/26800?src=pr&el=footer). Last update [129aaea...331c3a2](https://codecov.io/gh/pandas-dev/pandas/pull/26800?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/26800?src=pr&el=h1) Report
> Merging [#26800](https://codecov.io/gh/pandas-dev/pandas/pull/26800?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/129aaea46db94d5f8296bd5daeff9dc124b1a99e?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26800/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26800?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26800      +/-   ##
==========================================
+ Coverage   91.73%   91.76%   +0.02%     
==========================================
  Files         178      178              
  Lines       50774    50751      -23     
==========================================
- Hits        46578    46570       -8     
+ Misses       4196     4181      -15
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.35% <100%> (+0.03%)` | :arrow_up: |
| #single | `41.22% <60%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26800?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/26800/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `95.21% <ø> (+1.34%)` | :arrow_up: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/26800/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `94.38% <100%> (+0.25%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26800/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/26800/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.88% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26800/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.94% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26800?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/26800?src=pr&el=footer). Last update [129aaea...331c3a2](https://codecov.io/gh/pandas-dev/pandas/pull/26800?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jbrockmendel 