remove BlockManager.reindex
- [x] closes #19275
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19338?src=pr&el=h1) Report
> Merging [#19338](https://codecov.io/gh/pandas-dev/pandas/pull/19338?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5fdb9c0edef57da4b29a437eca84bad5f20719b7?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `50%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19338/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19338?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19338      +/-   ##
==========================================
+ Coverage   91.57%   91.59%   +0.02%     
==========================================
  Files         150      150              
  Lines       48705    48681      -24     
==========================================
- Hits        44600    44590      -10     
+ Misses       4105     4091      -14
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.96% <50%> (+0.02%)` | :arrow_up: |
| #single | `41.72% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19338?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/19338/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `95.3% <ø> (-0.02%)` | :arrow_down: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/19338/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `95.46% <ø> (+0.5%)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/19338/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.51% <50%> (-0.1%)` | :arrow_down: |
| [pandas/core/nanops.py](https://codecov.io/gh/pandas-dev/pandas/pull/19338/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbmFub3BzLnB5) | `96.68% <0%> (-0.02%)` | :arrow_down: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/19338/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.14% <0%> (-0.02%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19338?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/19338?src=pr&el=footer). Last update [5fdb9c0...e7eab6d](https://codecov.io/gh/pandas-dev/pandas/pull/19338?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks. FYI macosx builds appear to be delayed travis wide.