backends: Drop per-device idle monitors
We preserve the core one, which represents the union of all input devices. It might make sense to make this per-seat in the future, but certainly the per-device granularity is unused (at last!) and useless.
We preserve the core one, which represents the union of all input devices. It might make sense to make this per-seat in the future, but certainly the per-device granularity is unused (at last!) and useless.