BackgroundService: Remove __del__ method

We have no guarantee that the event loop is still running at the time
the __del__ method is called. In fact we do see errors in the
in the test cleanup phase exactly because of this.

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

src/frequenz/sdk/actor/_background_service.py

@@ -268,13 +268,6 @@ def __await__(self) -> collections.abc.Generator[None, None, None]:
         """
         return self.wait().__await__()
 
-    def __del__(self) -> None:
-        """Destroy this instance.
-
-        Cancel all running tasks spawned by this background service.
-        """
-        self.cancel("{self!r} was deleted")
-
     def __repr__(self) -> str:
         """Return a string representation of this instance.