remove unused args etal from np_datetime.c
What #18565 does for np_datetime_strings.c, this does for np_datetime.c

Getting rid of `meta` also gets rid of a potential problem noted in a comment that `meta->num` could potentially cause an overflow.  Since `meta->num` is 1 in all pandas usages, that pitfall doesn't apply.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18567?src=pr&el=h1) Report
> Merging [#18567](https://codecov.io/gh/pandas-dev/pandas/pull/18567?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bd9a3e06621e0b2bff3a7f4a6bf68c886b0db4c3?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18567/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18567?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18567      +/-   ##
==========================================
+ Coverage   91.44%   91.45%   +<.01%     
==========================================
  Files         157      157              
  Lines       51449    51449              
==========================================
+ Hits        47048    47051       +3     
+ Misses       4401     4398       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.32% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.6% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18567?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18567/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18567/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18567/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.25% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18567?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/18567?src=pr&el=footer). Last update [bd9a3e0...c9c643d](https://codecov.io/gh/pandas-dev/pandas/pull/18567?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 ping.  would like to close (most) existing before doing anything new. @jbrockmendel yep that is fine. just rebase & ping on anything you'd like me to review. (so rebase here). just rebased+pushed; no new edits. should be good to go.  #18565 may need rebasing after this; if not then it should be good to go too. thanks!