Remove AssertArg

See the commit b1099eca8f38ff5cfaf0901bb91cb6a22f909bc6 (Remove AssertArg and
AssertState) in PostgreSQL 16.

Author: Marina Polyakova

pg_variables.c

@@ -1582,7 +1582,7 @@ getMemoryTotalSpace(MemoryContext context, int level, Size *totalspace)
 	MemoryContext child;
 	MemoryContextCounters totals;
 
-	AssertArg(MemoryContextIsValid(context));
+	Assert(MemoryContextIsValid(context));
 
 	/* Examine the context itself */
 	memset(&totals, 0, sizeof(totals));