A long time ago, We added the image prepulling as a workaround due to
the overwhelming amount of flake caused by pulling during the tests.
This functionality has been broken for a while now when we switched to a
COS image where mounting `docker` binary into `busybox` stopped working.
So we just have dead code we should clean up.

Change-Id: I538171a5c1d9361eee7f9e0a99655b88b1721e3e