bpo-35125: remove inner callback on outer cancellation in asyncio shield (GH-10340)

bpo-35125: remove inner callback on outer cancellation in asyncio shield (GH-10340)

When the future returned by shield is cancelled, its completion callback of the
inner future is not removed. This makes the callback list of inner inner future
grow each time a shield is created and cancelled.

This change unregisters the callback from the inner future when the outer
future is cancelled.

https://bugs.python.org/issue35125