protected_call: remove left-over rootdir/dev mkdir

protected_call: remove left-over rootdir/dev mkdir

commit 797e12e8a5df246d8afc53b045e632977cdf0088 got rid of our "just
bind-mount the root /dev to the CT temporarily for some stuff" for
good a while ago (2015), but creating the /dev directory in the CT
root was kept, from what I can tell, by mistake.

This can be a problem if, whyever, the CT rootfs is not mounted, as
we then break a future mount as we create this /dev directory inside
what would be the CTs rootfs mount point. It is then not empty
anymore and a normal mount cannot happen, failing with "directory is
not empty"

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>