pgsql: Remove more obsolete comments about semaphores.

Remove more obsolete comments about semaphores.

Commit 6753333f stopped using semaphores as the sleep/wake mechanism for
heavyweight locks, but some obsolete references to that scheme remained
in comments.  As with similar commit 25b93a29, back-patch all the way.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CA%2BhUKGLafjB1uzXcy%3D%3D2L3cy7rjHkqOVn7qRYGBjk%3D%3DtMJE7Yg%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2f783380641db4cc598acd108186a446e4919290

Modified Files
--------------
src/backend/storage/lmgr/lwlock.c | 27 +++++++++------------------
1 file changed, 9 insertions(+), 18 deletions(-)