Remove the runtime lock

This primarily served to protect us against shutting down the
Libpod runtime while operations (like creating a container) were
happening. However, it was very inconsistently implemented (a lot
of our longer-lived functions, like pulling images, just didn't
implement it at all...) and I'm not sure how much we really care
about this very-specific error case?

Removing it also removes a lot of potential deadlocks, which is
nice.

[NO NEW TESTS NEEDED]

Signed-off-by: Matthew Heon <mheon@redhat.com>

libpod/runtime_volume_linux.go

@@ -21,9 +21,6 @@ import (
 
 // NewVolume creates a new empty volume
 func (r *Runtime) NewVolume(ctx context.Context, options ...VolumeCreateOption) (*Volume, error) {
-	r.lock.Lock()
-	defer r.lock.Unlock()
-
 	if !r.valid {
 		return nil, define.ErrRuntimeStopped
 	}