Remove unnecessary heap_tuple_needs_freeze argument.

The buffer argument hasn't been used since the function was first added
by commit bbb6e559c4.  The sibling heap_prepare_freeze_tuple function
doesn't have such an argument either.  Remove it.

src/backend/access/heap/heapam.c

@@ -7141,7 +7141,7 @@ heap_tuple_needs_eventual_freeze(HeapTupleHeader tuple)
  */
 bool
 heap_tuple_needs_freeze(HeapTupleHeader tuple, TransactionId cutoff_xid,
-						MultiXactId cutoff_multi, Buffer buf)
+						MultiXactId cutoff_multi)
 {
 	TransactionId xid;