A cleanup for: MDEV-16884 Remove tests for field_type() in Item_cache_temporal

It seems Item_cache_time::val_datetime_packed() is never caller in the
current code. Fixing it to use to_packed() rather than to_longlong() anyway.

sql/item.h

@@ -6419,7 +6419,7 @@ class Item_cache_time: public Item_cache_temporal
   longlong val_datetime_packed(THD *thd)
   {
     date_mode_t fuzzy= Datetime::comparison_flags_for_get_date();
-    return has_value() ? Datetime(thd, this, fuzzy).to_longlong() : 0;
+    return has_value() ? Datetime(thd, this, fuzzy).to_packed() : 0;
   }
   longlong val_time_packed(THD *thd)
   {