mount: drop duplicated unlink() call from the mount_dev_del()

There is no need to call unlink() there as mount_enum_drives() does it
too, soon after calling mount_dev_del(). Also mount_enum_drives() does
that for STATUS_EXPIRED - which is expected - as there is no unlink()
call when device expires.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>