Remove unused BusyBoxImage

The core e2e framework contains BusyBoxImage. The sub e2e "pod"
framework also contains it but not used at all.
So this removes this unused BusyBoxImage for code cleanup.

Author: Kenichi Omichi

test/e2e/framework/pod/resource.go

@@ -41,11 +41,6 @@ import (
 	imageutils "k8s.io/kubernetes/test/utils/image"
 )
 
-var (
-	// BusyBoxImage is the image URI of BusyBox.
-	BusyBoxImage = imageutils.GetE2EImage(imageutils.BusyBox)
-)
-
 // TODO: Move to its own subpkg.
 // expectNoErrorWithRetries to their own subpackages within framework.
 // expectNoError checks if "err" is set, and if so, fails assertion while logging the error.