Remove unused DeviceEventController methods

From #94 (closed) - I found a typo in impl_get_device_event_listeners(), as it was iterating through controller->key_listeners when the function was intended to be about mouse listeners. While digging for code that used that function, I discovered that none of the methods in DeviceEventController for "device events" are actually used by pyatspi2 nor orca. The code to monitor mouse events is used via Registry.RegisterEvent("mouse", ...).

This MR removes the relevant code in the following way:

• Remove the DBus methods from the XML.
• Remove the implementation code.
• Leave the public atspi_register_device_event_listener and atspi_deregister_device_event_listener as no-ops, to avoid an ABI break. We can remove them later. (They are never called anywhere.)

CC @mgorse for review

Fixes #94 (closed)