Remove obsolete comments about sempahores from proc.c.
Remove obsolete comments about sempahores from proc.c.

Commit 6753333f switched from a semaphore-based wait to a latch-based
wait for ProcSleep()/ProcWakeup(), but left behind some stray references
to semaphores.

Back-patch to 9.5.

Reviewed-by: Daniel Gustafsson, Michael Paquier
Discussion: a54909dbdb78ef9a6232fb5ebfb3ce8bd117eb6b@mail.gmail.com

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