Remove unused include

This was added in add5cf28d4 but was apparently never used.

Discussion: https://www.postgresql.org/message-id/flat/f84640e3-00d3-5abd-3f41-e6a19d33c40b@eisentraut.org

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"