Remove the option to service interrupts during PGSemaphoreLock().

Remove the option to service interrupts during PGSemaphoreLock().

The remaining caller (lwlocks) doesn't need that facility, and we plan
to remove ImmedidateInterruptOK entirely. That means that interrupts
can't be serviced race-free and portably anyway, so there's little
reason for keeping the feature.

Reviewed-By: Heikki Linnakangas