I really fail to see the point of this and git {blame, log -S} don't really
enlighten me on the reason for this as well. But I might be dense. The way I
see it the only thing this line achieves is causing trouble when the container
is started as root because the umount2() call will umount e.g.
/usr/lib/x86_64-linux-gnu/lxc in case it is a mountpoint on the host. Note,
this is because lxc_spawn() is still called in the hosts namespaces.

Closes https://github.com/lxc/lxd/#3255.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>