core: always try to reload not-found unit

This check was added in d904afc730. It would only
apply in the case where the cache hasn't been loaded yet. I think we pretty
much always have the cache loaded when we reach this point, but even if we
didn't, it seems better to try to reload the unit. So let's drop this check.

src/core/manager.c

@@ -1956,9 +1956,6 @@ bool manager_unit_cache_should_retry_load(Unit *u) {
         if (u->load_state != UNIT_NOT_FOUND)
                 return false;
 
-        if (u->manager->unit_cache_mtime == 0)
-                return false;
-
         /* The cache has been updated since the last time we tried to load the unit. There might be new
          * fragment paths to read. */
         if (u->manager->unit_cache_mtime != u->fragment_not_found_time)