refactor: Drop unused is_alive from AsyncUpdateProcessor

is_alive is never called on the async path; it was only mirrored from
the sync UpdateProcessor's BackgroundOperation base. The async interface
is defined fresh, so it omits the vestigial method.

Author: jsonbailey

ldclient/interfaces.py

@@ -481,12 +481,6 @@ async def stop(self):
         """
         pass
 
-    def is_alive(self) -> bool:
-        """
-        Returns whether the update processor is alive or not.
-        """
-        return True
-
     def initialized(self) -> bool:  # type: ignore[empty-body]
         """
         Returns whether the update processor has received feature flags and has initialized its feature store.