Remove never implemented wxDateTime::IsGregorianDate().

Remove never implemented wxDateTime::IsGregorianDate().

This method was declared and even documented but never actually implemented,
and GregorianAdoption enum used by it had many incorrectly defined elements.

Just remove both the method and the enum instead of fixing/implementing them
as they are not that important (as witnessed by the fact that nobody has
complained about the method being missing since almost 15 years).

Closes #15400.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74648 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775