Remove code handling FORCE_NULL and FORCE_NOT_NULL for COPY TO

These two options are only available with COPY FROM, so the extra logic
in charge of checking the validity of the attributes given has no
purpose.

Author: Zhang Mingli
Reviewed-by: Richard Guo, Kyotaro Horiguchi
Discussion: https://postgr.es/m/F28F0B5A-766F-4D33-BF44-43B3A052D833@gmail.com

src/backend/commands/copyto.c

@@ -608,52 +608,6 @@ BeginCopyTo(ParseState *pstate,
 		}
 	}
 
-	/* Convert FORCE_NOT_NULL name list to per-column flags, check validity */
-	cstate->opts.force_notnull_flags = (bool *) palloc0(num_phys_attrs * sizeof(bool));
-	if (cstate->opts.force_notnull)
-	{
-		List	   *attnums;
-		ListCell   *cur;
-
-		attnums = CopyGetAttnums(tupDesc, cstate->rel, cstate->opts.force_notnull);
-
-		foreach(cur, attnums)
-		{
-			int			attnum = lfirst_int(cur);
-			Form_pg_attribute attr = TupleDescAttr(tupDesc, attnum - 1);
-
-			if (!list_member_int(cstate->attnumlist, attnum))
-				ereport(ERROR,
-						(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
-						 errmsg("FORCE_NOT_NULL column \"%s\" not referenced by COPY",
-								NameStr(attr->attname))));
-			cstate->opts.force_notnull_flags[attnum - 1] = true;
-		}
-	}
-
-	/* Convert FORCE_NULL name list to per-column flags, check validity */
-	cstate->opts.force_null_flags = (bool *) palloc0(num_phys_attrs * sizeof(bool));
-	if (cstate->opts.force_null)
-	{
-		List	   *attnums;
-		ListCell   *cur;
-
-		attnums = CopyGetAttnums(tupDesc, cstate->rel, cstate->opts.force_null);
-
-		foreach(cur, attnums)
-		{
-			int			attnum = lfirst_int(cur);
-			Form_pg_attribute attr = TupleDescAttr(tupDesc, attnum - 1);
-
-			if (!list_member_int(cstate->attnumlist, attnum))
-				ereport(ERROR,
-						(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
-						 errmsg("FORCE_NULL column \"%s\" not referenced by COPY",
-								NameStr(attr->attname))));
-			cstate->opts.force_null_flags[attnum - 1] = true;
-		}
-	}
-
 	/* Use client encoding when ENCODING option is not specified. */
 	if (cstate->opts.file_encoding < 0)
 		cstate->file_encoding = pg_get_client_encoding();