CLN: _try_coerce_result, redundant dtypes.missing method
Started this branch with the idea of making DatetimeBlock, TimedeltaBlock, DatetimeTZBlock define _try_coerce_result using self._holder._from_sequence (which I still think is worthwhile, cc @TomAugspurger ) and got side-tracked.  This ends up just being some cleanup and removal of a redundant method.

Tiny perf bump expected from optimizing NaT checks in _libs.missing.  Further improvements/cleanups may be possible pending #24607.

The object-dtype cases in try_coerce_result are no longer hit following #24606.