honor enum length in all cases

honor enum length in all cases

The :paramref:`.Enum.length` parameter, which sets the length of the
``VARCHAR`` column for non-native enumeration types, is now used
unconditionally when emitting DDL for the ``VARCHAR`` datatype, including
when the :paramref:`.Enum.native_enum` parameter is set to ``True`` for
target backends that continue to use ``VARCHAR``. Previously the parameter
would be erroneously ignored in this case. The warning previously emitted
for this case is now removed.

Fixes: #7791
Change-Id: I91764546b56e9416479949be8a118cdc91ac5ed9