CLN: Remove Unneeded BlockManager methods
BlockManager.get_scalar is never used outside of tests, 
BlockManager._is_indexed_like is only ever called from `merge`, which is only called once in Panel, before which the indexed_like check is already done.

Also BlockPlacements.iadd was more complicated than it needed to be, so tore out bits of that.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22002?src=pr&el=h1) Report
> Merging [#22002](https://codecov.io/gh/pandas-dev/pandas/pull/22002?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/171076cc4f37af3f1d050237d0aa3e8000d2bca3?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22002/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22002?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22002      +/-   ##
==========================================
- Coverage      92%   91.99%   -0.01%     
==========================================
  Files         170      170              
  Lines       50609    50594      -15     
==========================================
- Hits        46561    46546      -15     
  Misses       4048     4048
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.4% <ø> (-0.01%)` | :arrow_down: |
| #single | `42.2% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22002?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/22002/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `96.47% <ø> (-0.06%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22002?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/22002?src=pr&el=footer). Last update [171076c...4504ccf](https://codecov.io/gh/pandas-dev/pandas/pull/22002?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks, always +1 on removing code!