Remove unused include

This was added in add5cf2 but was apparently never used.

Discussion: https://www.postgresql.org/message-id/flat/[email protected]

Author: petere

src/backend/catalog/index.c

@@ -74,7 +74,6 @@
 #include "storage/procarray.h"
 #include "storage/smgr.h"
 #include "utils/builtins.h"
-#include "utils/datum.h"
 #include "utils/fmgroids.h"
 #include "utils/guc.h"
 #include "utils/inval.h"