CLN: remove is_null_datetimelike
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

No longer needed following #44507
> can u perf check things, though wouldn't anticpate any slowdowns

```
In [3]: from pandas._libs.missing import checknull
In [4]: %timeit checknull("foo")
74.2 ns ± 0.39 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)  # <- master
73.4 ns ± 0.323 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)  # <- PR
```