libcontainer/system: remove alias for deprecated RunningInUserNS
libcontainer/system: remove alias for deprecated RunningInUserNS

These were deprecated and moved; the stubs were included in the
last two (rc94, rc95) releases, so external consumers would have
the chance to update their code.

Removing this so that this doesn't get into v1.0.0 GA

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

rhatdan/runc,kolyshkin/runc,dqminh/runc,kolyshkin/runc,opencontainers/runc,rhatdan/runc,kolyshkin/runc,kolyshkin/runc,rhatdan/runc,opencontainers/runc,opencontainers/runc,dqminh/runc,rhatdan/runc,dqminh/runc,opencontainers/runc,dqminh/runc