CLN: Remove unused core.internals methods
De-duplicate some bits of DatetimeBlock and DatetimeTZBlock.

- [x] closes #19243
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19250?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@ae96187`). [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/19250/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19250?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19250   +/-   ##
=========================================
  Coverage          ?   91.55%           
=========================================
  Files             ?      150           
  Lines             ?    48713           
  Branches          ?        0           
=========================================
  Hits              ?    44599           
  Misses            ?     4114           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.92% <100%> (?)` | |
| #single | `41.74% <66.66%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19250?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/19250/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.95% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19250?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19250?src=pr&el=footer). Last update [ae96187...59b3e60](https://codecov.io/gh/pandas-dev/pandas/pull/19250?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Travis error looks like OSX; I'll sit tight until advised otherwise. my basic point is that the inheritance order needs to change for this, IOW NonConsolidated needs to be moved to the end of the MRO (and not the beginning). Then this will simply work (of course that might break other things) ping on green. That makes sense; thanks for explaining. thanks!