optimize _expired_computed_vars to one use of getattribute (#5946)

Author: adhami3310

reflex/state.py

@@ -1999,8 +1999,8 @@ def _expired_computed_vars(self) -> set[str]:
         """
         return {
             cvar
-            for cvar in self.computed_vars
-            if self.computed_vars[cvar].needs_update(instance=self)
+            for cvar, cvar_obj in self.computed_vars.items()
+            if cvar_obj.needs_update(instance=self)
         }
 
     def _dirty_computed_vars(