mount_helper: drop unused module

There should be no more users of this after devmode-test converted
to chromite, so drop it!

BUG=chromium:1000719
TEST=CQ passes

Change-Id: I3867761accdc1a67a67cf4f900df37e65dbc2568
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/1808026
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
1 file changed