Add delay kwarg to message.delete()

Refactor delete_after kwargs to use new delay kwarg

discord/abc.py

@@ -784,13 +784,7 @@ class Messageable(metaclass=abc.ABCMeta):
 
         ret = state.create_message(channel=channel, data=data)
         if delete_after is not None:
-            async def delete():
-                await asyncio.sleep(delete_after, loop=state.loop)
-                try:
-                    await ret.delete()
-                except HTTPException:
-                    pass
-            asyncio.ensure_future(delete(), loop=state.loop)
+            await ret.delete(delay=delete_after)
         return ret
 
     async def trigger_typing(self):