Don't increment dirty on expireIfNeeded() as natural expires are not...

Don't increment dirty on expireIfNeeded() as natural expires are not considered database changes. This will avoid useless read-only commands in the AOF file as a result of, for instance, a GET operation triggering an expirIfNeeded() call resulting in an expired key removed.

src/db.c

@@ -468,7 +468,6 @@ int expireIfNeeded(redisDb *db, robj *key) {
 
     /* Delete the key */
     server.stat_expiredkeys++;
-    server.dirty++;
     propagateExpire(db,key);
     return dbDelete(db,key);
 }