[python] remove/deprecate Monitor.onAbortRequested()

I fail to see any use case for this. It's almost always used incorrectly causing only confusion believing it will reliably be called when abort is requested.

If the original PR #1053 is anything to judge by, it was added with the false premise that it could "replace[..] checking for xbmc.abortRequested in a while loop". Since the main thread have to be kept alive and non-blocking to receive callbacks, you will need another mechanism to wait for the callback itself.