ref(transport): Move flush_async from ABC

Move flush_async down to the specific async transport subclass. This makes more sense anyway, as
this will only be required by the async transport. If more async transports are expected,
another shared superclass can be created.

GH-4568

Author: srothh

sentry_sdk/transport.py

@@ -107,19 +107,6 @@ def flush(
         """
         return None
 
-    async def flush_async(
-        self: Self,
-        timeout: float,
-        callback: Optional[Any] = None,
-    ) -> None:
-        """
-        Send out current events within `timeout` seconds. This method needs to be awaited for blocking behavior.
-
-        The default implementation is a no-op, since this method may only be relevant to some transports.
-        Subclasses should override this method if necessary.
-        """
-        return None
-
     def kill(self: Self) -> None:
         """
         Forcefully kills the transport.