[db] Add function to cleanup the 'groups' table (artists and albums)

Allows removing of item no longer referenced from the files table

src/db.h

@@ -617,6 +617,9 @@ db_pl_enable_bycookie(uint32_t cookie, char *path);
 int
 db_groups_clear(void);
 
+int
+db_groups_cleanup();
+
 int
 db_group_persistentid_byid(int id, int64_t *persistentid);