Remove unnecessary include of "utils/fmgroids.h"

In initsplan.c, no macros for built-in function OIDs are used, so this
include is unnecessary and can be removed.  This was my oversight in
commit 8e1185910.

Discussion: https://postgr.es/m/CAMbWs4_-sag-cAKrLJ+X+5njL1=oudk=+KfLmsLZ5a2jckn=kg@mail.gmail.com

src/backend/optimizer/plan/initsplan.c

@@ -32,7 +32,6 @@
 #include "optimizer/restrictinfo.h"
 #include "parser/analyze.h"
 #include "rewrite/rewriteManip.h"
-#include "utils/fmgroids.h"
 #include "utils/lsyscache.h"
 #include "utils/rel.h"
 #include "utils/typcache.h"