Revert "Mount proc with the hidepid=2 option"

We were implicitly using proc:mixed to mount /proc/sys as read-only.
Without the automount the whole /proc was mounted as read-write which
we want to avoid.

I'm unable to replicate the proc:mixed logic with lxc.mount.entry
so revert this change until a solution is found.

This reverts commit f5543df68651277042b156f19f5b3804c297f2a7.
Fixes: #642