ref(worker): Move worker flush_async from Worker ABC

Move the flush_async down to the concrete subclass to not break existing testing. This makes sense,
as this will only really be needed by the async worker anyway and therefore is not shared logic.

GH-4578

Author: srothh

sentry_sdk/worker.py

@@ -45,18 +45,6 @@ def flush(
         """
         return None
 
-    async def flush_async(
-        self, timeout: float, callback: Optional[Callable[[int, float], None]] = None
-    ) -> None:
-        """
-        Flush the worker.
-
-        This method can be awaited until the worker has flushed all events or the specified timeout is reached.
-        Default implementation is a no-op, since this method may only be relevant to some workers.
-        Subclasses should override this method if necessary.
-        """
-        return None
-
     @abstractmethod
     def full(self) -> bool:
         pass