automount: turn the separated automount binary back into a plugin

It was separated for using only in GNOME fallback mode, but now it's always used in Cinnamon. There is no reason to keep it as a separated binary.

Basically, this patch reverts back this commit:
https://git.gnome.org/browse/gnome-settings-daemon/commit/?id=796958e94a72691fce1fcb50211895e5b9a4d646

At the same time when this patch applied, 'cinnamon-fallback-mount-helper' should be removed from the 'RequiredComponents' line in Cinnamon session files.