sftp: Don't call g_vfs_backend_force_unmount multiple times

If the remote connection(s) are closed (e.g. the remote host is
rebooted), ensure that g_vfs_backend_force_unmount is only called once.

This avoids a warning like the following:
forced_unregister_mount_callback
forced_unregister_mount_callback

** (process:12472): WARNING **: Error unregistering mount: Mountpoint
not registered (g-io-error-quark, 16)

Additionally, remove some backend member variables.

https://bugzilla.gnome.org/show_bug.cgi?id=753311