virt: drop /proc/1/sched hack

virt: drop /proc/1/sched hack

On really old kernels (< 4.14+) a bug in /proc/1/sched handling in the
kernel could be used to determine whether we are running in a PID
namespace. This hasn't worked for a long time, and there's little point
in making things work on old kernels we can't make work on current
kernels, hence let's drop that old cruft.

See: #8153