pgsql: Remove unused ShutdownLatchSupport() function

Remove unused ShutdownLatchSupport() function

The only caller was removed in commit 80a8f95b3b. I don't foresee
needing it any time soon, and I'm working on some big changes in this
area, so let's remove it out of the way.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/8a507fb6-df28-49d3-81a5-ede180d7f0fb@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a98e4dee63ce594ed32d1b004385d6ffa8b57dc5

Modified Files
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src/backend/storage/ipc/latch.c | 27 ---------------------------
src/include/storage/latch.h     |  1 -
2 files changed, 28 deletions(-)