udisks2: Disconnect signal handlers before freeing data
After the commit 1b80450a, the udisks2 volume monitor crashes sometimes after
showing the unmount notification. This is because the UnmountNotifyData
struct is freed, but signal handlers are still connected. Let's disconnect
the signal handlers to avoid those crashes caused by invalid reads.
Related: https://bugzilla.redhat.com/show_bug.cgi?id=2219172