libnetwork: remove Controller.Sandboxes as it's no longer used

The Controller.Sandboxes method was used by some SandboxWalkers. Now
that those have been removed, there are no longer any consumers of this
method, so let's remove it for now.

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

libnetwork/controller.go

@@ -966,24 +966,6 @@ func (c *Controller) NewSandbox(containerID string, options ...SandboxOption) (*
 	return sb, nil
 }
 
-// Sandboxes returns the list of Sandbox(s) managed by this controller.
-func (c *Controller) Sandboxes() []*Sandbox {
-	c.mu.Lock()
-	defer c.mu.Unlock()
-
-	list := make([]*Sandbox, 0, len(c.sandboxes))
-	for _, s := range c.sandboxes {
-		// Hide stub sandboxes from libnetwork users
-		if s.isStub {
-			continue
-		}
-
-		list = append(list, s)
-	}
-
-	return list
-}
-
 // GetSandbox returns the Sandbox which has the passed id.
 //
 // It returns an [ErrInvalidID] when passing an invalid ID, or an