Remove heapam.h include made superfluous by b60c397.

Noticed this while working on another patch.

Author: Andres Freund

Author: anarazel

src/backend/optimizer/prep/prepunion.c

@@ -23,7 +23,6 @@
  */
 #include "postgres.h"
 
-#include "access/heapam.h"
 #include "access/htup_details.h"
 #include "access/sysattr.h"
 #include "catalog/partition.h"