Remove useless bms_copy step in RelationGetIndexAttrBitmap.

Seems to be from a bad case of copy-and-paste-itis in commit 665d1fad9.
It wouldn't be quite so annoying if it didn't contradict the comment
half a dozen lines above.

David Rowley

Discussion: https://postgr.es/m/CAKJS1f95Dyf8Qkdz4W+PbCmT-HTb54tkqUCC8isa2RVgSJ_pXQ@mail.gmail.com

src/backend/utils/cache/relcache.c

@@ -4840,7 +4840,7 @@ restart:
 		case INDEX_ATTR_BITMAP_KEY:
 			return uindexattrs;
 		case INDEX_ATTR_BITMAP_PRIMARY_KEY:
-			return bms_copy(relation->rd_pkattr);
+			return pkindexattrs;
 		case INDEX_ATTR_BITMAP_IDENTITY_KEY:
 			return idindexattrs;
 		default: