media-keys: Disable rfkill keys handling

If we handle rfkill input keys without telling rfkill, we end up
handling those keys in gnome-settings-daemon, as well as in the kernel's
rfkill-input driver. As handling all the different keys would require
more work than possible in a stable release, we'll let rfkill-input
handle this, and lose the OSD for now.

https://bugzilla.gnome.org/show_bug.cgi?id=760517