Ignore join_transaction_mode when bind is an engine

Ignore join_transaction_mode when bind is an engine

Ignore :paramref:`_orm.Session.join_transaction_mode` in all cases when
the bind provided to the :class:`_orm.Session` is an
class:`_engine.Engine`.
Previously if an event that executed before the session logic,
like :meth:`_engine.ConnectionEvents.engine_connect`,
left the connection with an active transaction, the
paramref:`_orm.Session.join_transaction_mode` behavior took
place, leading to a surprising behavior.

Fixes: #11163
Change-Id: I10147876d07352f2dab898d615e98a9acd6eb91b