Remove BotBase.closing_tasks

Doing a naive git log --grep closing_tasks on the bot repo I found these two places it was used:

python-discord/bot@429cc86 - Reddit cog to revoke the access token on unload, which has since been moved to lance (without this feature).
python-discord/bot@f4004d8 - Silence cog, which has been removed since.

Since this list of tasks to close when closing the Discord connection is no longer used, it has been removed from BotBase.

Author: ChrisLovering

botcore/_bot.py

@@ -109,9 +109,6 @@ def _connect_statsd(
                 attempt + 1
             )
 
-        # All tasks that need to block closing until finished
-        self.closing_tasks: list[asyncio.Task] = []
-
     async def load_extensions(self, module: types.ModuleType) -> None:
         """Load all the extensions within the given module and save them to ``self.all_extensions``."""
         self.all_extensions = walk_extensions(module)
@@ -240,10 +237,6 @@ async def close(self) -> None:
             with suppress(Exception):
                 await self.remove_cog(cog)
 
-        # Wait until all tasks that have to be completed before bot is closing is done
-        log.trace("Waiting for tasks before closing.")
-        await asyncio.gather(*self.closing_tasks)
-
         # Now actually do full close of bot
         await super().close()