Fix mistake in new GUC tables source

Commit 6359989654 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.

src/backend/utils/misc/guc_parameters.dat

@@ -2295,7 +2295,6 @@
   boot_val => 'DEFAULT_DEBUG_DISCARD_CACHES',
   min => 'MIN_DEBUG_DISCARD_CACHES',
   max => 'MAX_DEBUG_DISCARD_CACHES',
-  ifdef => 'DISCARD_CACHES_ENABLED',
 },
 
 { name => 'client_connection_check_interval', type => 'int', context => 'PGC_USERSET', group => 'CONN_AUTH_TCP',