Fix mistake in new GUC tables source

Fix mistake in new GUC tables source

Commit 63599896545 had it so that the parameter "debug_discard_caches"
did not exist unless DISCARD_CACHES_ENABLED was defined (typically via
enabling asserts).  This was a mistake, it did not correspond to the
prior setup.  Several tests use this parameter, so they were now
failing if you did not have asserts enabled.