Ensure the "orm" plugin is used unconditionally for bundles

Ensure the "orm" plugin is used unconditionally for bundles

This also introduces that the "orm" plugin may be used
when the plugin_subject is None.

Fixed regression where the :paramref:`.Bundle.single_entity` flag would
take effect for a :class:`.Bundle` even though it were not set.
Additionally, this flag is legacy as it only makes sense for the
:class:`_orm.Query` object and not 2.0 style execution.  a deprecation
warning is emitted when used with new-style execution.

Fixes: #5702
Change-Id: If9f43365b9d966cb398890aeba2efa555658a7e5