Remove dead code.

These memory context switches are useless in the wake of commit
1cff1b95.  Noted by Jesper Pedersen.

Discussion: https://postgr.es/m/f078ce63-9e04-0f3e-d200-d7ee66279abe@redhat.com

src/backend/commands/async.c

@@ -693,15 +693,8 @@ pg_listening_channels(PG_FUNCTION_ARGS)
 	/* stuff done only on the first call of the function */
 	if (SRF_IS_FIRSTCALL())
 	{
-		MemoryContext oldcontext;
-
 		/* create a function context for cross-call persistence */
 		funcctx = SRF_FIRSTCALL_INIT();
-
-		/* switch to memory context appropriate for multiple function calls */
-		oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
-
-		MemoryContextSwitchTo(oldcontext);
 	}
 
 	/* stuff done on every call of the function */