Simplify core state cache clear (#114694)

same as #113136 but for core

homeassistant/core.py

@@ -440,8 +440,7 @@ class HomeAssistant:
         """Set the current state."""
         self.state = state
         for prop in ("is_running", "is_stopping"):
-            with suppress(AttributeError):
-                delattr(self, prop)
+            self.__dict__.pop(prop, None)
 
     def start(self) -> int:
         """Start Home Assistant.