libcontainerd/supervisor: remove unused RWMutex
libcontainerd/supervisor: remove unused RWMutex

This RWMutex was added in 9c4570a958df42d1ad19364b1a8da55b891d850a, and used in
the `remote.Client()` method. Commit dd2e19ebd58cd8896d79b4b8db61144b93717b33
split the code for client and daemon, but did not remove the mutex.

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

konstruktoid/docker-upstream,mahak/docker,trebonian/docker,jimjag/docker,moby/moby,jimjag/docker,cpuguy83/docker,moby/moby,mahak/docker,trebonian/docker,jimjag/docker,jimjag/docker,moul/docker,konstruktoid/docker-upstream,konstruktoid/docker-upstream,Mokolea/docker,trebonian/docker,Ramzec/docker,moby/moby,moby/moby,moul/docker,thaJeztah/docker,mahak/docker,konstruktoid/docker-upstream,cpuguy83/docker,thaJeztah/docker,Mokolea/docker,cpuguy83/docker,cpuguy83/docker,thaJeztah/docker,Ramzec/docker,mahak/docker,Mokolea/docker,moul/docker,Mokolea/docker,trebonian/docker,Ramzec/docker,Ramzec/docker,moul/docker,thaJeztah/docker