pgsql: 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: https://postgr.es/m/CA+hUKGLs5H6zhmgTijZ1OaJvC1sG0=AFXc1aHuce32tKiQrdEA@mail.gmail.com

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/df098c371b882eeafa168501f0d20330c9a186fc

Modified Files
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src/backend/storage/lmgr/proc.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)