omit_join=True is not supported

omit_join=True is not supported

The :paramref:`.relationship.omit_join` flag was not intended to be
manually set to True, and will now emit a warning when this occurs.  The
omit_join optimization is detected automatically, and the ``omit_join``
flag was only intended to disable the optimization in the hypothetical case
that the optimization may have interfered with correct results, which has
not been observed with the modern version of this feature.   Setting the
flag to True when it is not automatically detected may cause the selectin
load feature to not work correctly when a non-default primary join
condition is in use.

Fixes: #4954
Change-Id: I3afebefb13ddaf8f74c5c2b9e6e6a1a261ac5629