CreateUserMapping has a recordDependencyOnCurrentExtension call that's
been there since extensions were introduced (very possibly my fault).
However, there's no support anywhere else for user mappings as members
of extensions, nor are they listed as a possible member object type in
the documentation.  Nor does it really seem like a good idea for user
mappings to belong to extensions when roles don't.  Hence, remove the
bogus call.

(As we saw in bug #15310, the lack of any pg_dump support for this case
ensures that any such membership record would silently disappear during
pg_upgrade.  So there's probably no need for us to do anything else
about cleaning up after this mistake.)

Discussion: https://postgr.es/m/27952.1533667213@sss.pgh.pa.us