[python] deprecate Monitor.onAbortRequested

This is almost always used incorrectly causing only confusion believing it will be reliably called when abort is requested. I fail to see any use case for it as it is now.

According to the original PR #1053, it was added so  it could "replace[..] checking for xbmc.abortRequested in a while loop". This could never work since the main thread have to be kept alive and non-blocking to receive callbacks, and for that you will need another mechanism to wait for the callback itself.