Revert "Support using BackgroundService as a *mixin*"

Since `Reconfigurable` was removed we don't need this anymore. The added
possibility to use as a mixin doesn't seem to justify the loss of safety
of adding arbitrary keyword arguments to the constructor.

This reverts commit 5eb64165e8603bb50a5c0eb661de7c5770022cb2.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/sdk/actor/_background_service.py

@@ -69,20 +69,15 @@ async def main() -> None:
         ```
     """
 
-    def __init__(self, *, name: str | None = None, **kwargs: Any) -> None:
+    def __init__(self, *, name: str | None = None) -> None:
         """Initialize this BackgroundService.
 
         Args:
             name: The name of this background service. If `None`, `str(id(self))` will
                 be used. This is used mostly for debugging purposes.
-            **kwargs: Additional keyword arguments to be passed to the parent class
-                constructor. This is only provided to allow this class to be used as
-                a mixin alonside other classes that require additional keyword
-                arguments.
         """
         self._name: str = str(id(self)) if name is None else name
         self._tasks: set[asyncio.Task[Any]] = set()
-        super().__init__(**kwargs)
 
     @abc.abstractmethod
     def start(self) -> None: