Remove special rule for TypeDecorator of TypeDecorator

Remove special rule for TypeDecorator of TypeDecorator

Removing this check for "TypeDecorator" in impl seems to not
break anything and allows TypeDecorator.with_variant() to
work correctly.   The line has been traced back to 2007 and
does not appear to have relevance today.

Fixed bug where making use of the :meth:`.TypeEngine.with_variant` method
on a :class:`.TypeDecorator` type would fail to take into account the
dialect-specific mappings in use, due to a rule in :class:`.TypeDecorator`
that was instead attempting to check for chains of :class:`.TypeDecorator`
instances.

Fixes: #5816
Change-Id: Ic86d9d985810e3050f15972b4841108acca2fa3e