libmount: don't base GC startup on no-mtab context

When mtab is a symlink to /proc/mounts, libmount uses /run/mount/utab to
store filesystem-specific mount attributes, making this check
unnecessary. Worse, it now breaks nilfs_cleanerd under systemd 216+
since it always passes `-n` to the mount command.

Signed-off-by: Dan McGee <dan@archlinux.org>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>