Remove the Label alias in pandas._typing.

[simonjayhawkins]

in mypy 0.790 bugfix (open PR to update #37108)

Make None compatible with Hashable (PR 9371)

we have an alias in pandas._typing for Label to work around this bug.

I propose that we now remove the Label alias for two reasons.

1. It has been the source of some confusion, especially when using Optional[Label]
2. for the API, users would know what Hashable is without needing to look up the alias.