mount: remove obsolete -n

mount: remove obsolete -n

It seems that systemd still uses legacy -n option. The option has been
originally designed to avoid write to /etc/mtab during boot when root
FS is not ready or read-only.

This is not necessary for long time, because /etc/mtab is not a real
file (it's symlink) and write to the file is impossible. All utils
should be able to detect the symlink and ignore mtab. This concept is
supported for very long time before systemd.

The userspase mount options are currently maintained by libmount
(mount(8) and mount.nfs) in /run/mount) which is tmpfs initialized
during early boot.