CLN: Remove np_datetime64_compat and np_array_datetime64_compat

[phofl]

We probably can remove both checks, see #39092

[phofl]

Hm probably can not remove this.

np.datetime64("2014-02-01 00:00Z")

still raising

DeprecationWarning: parsing timezone aware datetimes is deprecated; this will raise an error in the future
  np.datetime64("2014-02-01 00:00Z")

Unless we want to remove the Z?

cc @jreback

[jbrockmendel]

We only use it in tests and AFAICT all of those places we'd be better off just stripping the "Z", yes.