CLN: remove unused `mgr` kwarg
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/internals/blocks.py`](https://github.com/jbrockmendel/pandas/blob/62f9d3efe226b31bdf788eadcebd5f77020e456b/pandas/core/internals/blocks.py) !

 - There are no PEP8 issues in the file [`pandas/core/internals/managers.py`](https://github.com/jbrockmendel/pandas/blob/62f9d3efe226b31bdf788eadcebd5f77020e456b/pandas/core/internals/managers.py) !

 looks fine, ping on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23260?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@779aeac`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23260/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23260?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #23260   +/-   ##
=========================================
  Coverage          ?   92.22%           
=========================================
  Files             ?      169           
  Lines             ?    50904           
  Branches          ?        0           
=========================================
  Hits              ?    46947           
  Misses            ?     3957           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.65% <100%> (?)` | |
| #single | `42.29% <85.71%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23260?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/23260/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `95.73% <100%> (ø)` | |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/23260/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `94.08% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23260?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/23260?src=pr&el=footer). Last update [779aeac...63dadd1](https://codecov.io/gh/pandas-dev/pandas/pull/23260?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 All green.