Cleaning up Kommodity

Replacing deprecated functions, obsolete ones from version 2.22 of Gio.
Removing unused variables.

The function g_volume_monitor_adopt_orphan_mount is obsolete and must
be reimplemented as explained here:

https://developer.gnome.org/gio/2.29/GVolumeMonitor.html#g-volume-monitor-Adopt-orphan-mount