PostgreSQL: Improve dropping of foreign key constraint

PostgreSQL: Improve dropping of foreign key constraint

Change I2abd650c8ce83c5b725aec054 to fix bug T76254 was
inefficient upon subsequent updates, as update.php would
first add the constraint, and then remove it again.

Remove the older code responsible for adding the doomed
constraint.

Also, make the "do nothing" output message more consistent
with other messages of the same nature.

Change-Id: Id6a05c1234fbdc5849480d13fb7d6476eac52857