remove unnecessary include in src/backend/commands/policy.c

hi.

in src/backend/commands/policy.c, i found that
#include "access/htup.h"
#include "access/htup_details.h"
#include "catalog/catalog.h"
#include "nodes/pg_list.h"
#include "parser/parse_node.h"
#include "utils/array.h"

is not necessary "include", so I removed it.

we can also remove
#include "access/relation.h"
replace relation_open to table_open, since we already did relkind check in
RangeVarCallbackForPolicy.