Remove partial and undocumented GRANT .. FOREIGN TABLE support.

Instead, foreign tables are treated just like views: permissions can
be granted using GRANT privilege ON [TABLE] foreign_table_name TO role,
and revoked similarly.  GRANT/REVOKE .. FOREIGN TABLE is no longer
supported, just as we don't support GRANT/REVOKE .. VIEW.  The set of
accepted permissions for foreign tables is now identical to the set for
regular tables, and views.

Per report from Thom Brown, and subsequent discussion.