Remove unused from datetime.pxd, check for fastpath in ensure_datetime64ns
Still a hodgepodge, but a somewhat smaller hodgepodge.

Same geopandas error.  Looks like its showing up in a couple of PRs (although a couple also have problems I need to fix). # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=h1) Report
> Merging [#18453](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e6eac0b308af9869ee123caa8c256bd8a7cc126b?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18453/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18453      +/-   ##
==========================================
- Coverage   91.35%   91.34%   -0.02%     
==========================================
  Files         163      163              
  Lines       49695    49695              
==========================================
- Hits        45401    45392       -9     
- Misses       4294     4303       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.14% <ø> (ø)` | :arrow_up: |
| #single | `39.66% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18453?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/18453?src=pr&el=footer). Last update [e6eac0b...6e391d8](https://codecov.io/gh/pandas-dev/pandas/pull/18453?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/18453?src=pr&el=h1) Report
> Merging [#18453](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e6eac0b308af9869ee123caa8c256bd8a7cc126b?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18453/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18453      +/-   ##
==========================================
- Coverage   91.35%   91.31%   -0.05%     
==========================================
  Files         163      163              
  Lines       49695    49695              
==========================================
- Hits        45401    45380      -21     
- Misses       4294     4315      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.11% <ø> (-0.03%)` | :arrow_down: |
| #single | `39.66% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.44% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18453?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/18453?src=pr&el=footer). Last update [e6eac0b...6e391d8](https://codecov.io/gh/pandas-dev/pandas/pull/18453?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 prob should just blow away src/datetime.pxd > prob should just blow away src/datetime.pxd

Definitely coming up, yah.