Move enable_eagerloads(False) out of _from_self() into count()

Move enable_eagerloads(False) out of _from_self() into count()

Fixed regression where calling upon :meth:`_orm.Query.count` in conjunction
with a loader option such as :func:`_orm.joinedload` would fail to ignore
the loader option. This is a behavior that has always been very specific to
the :meth:`_orm.Query.count` method; an error is normally raised if a given
:class:`_orm.Query` has options that don't apply to what it is returning.

Specifically, the call to enable_eagerloads(False) inside of
_from_self() is not needed as loader options are now not invoked
for subqueries.  Instead, set enable_eagerloads(False) in the
count() method itself, so that these options won't be considered
in this specific case.

Fixes: #6052
Change-Id: I0059ed3fb06156ef4116fd015cbef6f89808e8ef