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_img.go

@@ -25,9 +25,6 @@ import (
 // we can use the libpod-internal removal logic.
 func (r *Runtime) RemoveContainersForImageCallback(ctx context.Context) libimage.RemoveContainerFunc {
 	return func(imageID string) error {
-		r.lock.Lock()
-		defer r.lock.Unlock()
-
 		if !r.valid {
 			return define.ErrRuntimeStopped
 		}