PostgreSQL: Drop unneeded foreign key constraint

PostgreSQL: Drop unneeded foreign key constraint

Change I1c7f3a84f10df05d6b37dccbad4c8232edf51580 causes
an existing foreign key assumption (under PostgreSQL) to be
violated upon deleting a page.  This foreign key assumption does not
explicitly exist in MySQL, and is not implied via documentation.  So
it was probably never needed in the first place.

Don't create the foreign key constraint in PostgreSQL, and drop it
if it already exists when running update.php.

The constraint was previously created with an implicit name, so
drop the constraint involving the specified column name (rc_cur_id),
rather than hard-coding the name of the constraint itself.

This bug probably exists under Oracle and MSSQL as well, but no attempt
was made to address it there.

Bug: T76254
Change-Id: I2abd650c8ce83c5b725aec0545fff14a927a305a