text: Title: BUG Error while deleting a task
  Body: I am trying to delete a task, but Rocketry returns a KeyError.
  
  ```python
  import asyncio
  from rocketry import Rocketry
  from rocketry.conds import secondly
  
  app = Rocketry()
  
  @app.task(secondly, name='foo')
  async def foo():
      ...
  
  
  async def main():
      asyncio.create_task(app.serve())
      app.session.remove_task('foo')
      # app.session.tasks.remove('foo')
  
  
  if __name__ == '__main__':
      asyncio.run(main())
  
  ```
  