(R17) chromeos-firmware: Don't preserve bitmaps for updater v3+.

Since v3, firmware can render HWID directly so official firmware should carry
its own bitmaps; no need to preserve bitmaps anymore.

Cherry-picked: https://gerrit.chromium.org/gerrit/13331

BUG=chrome-os-partner:7328,chrome-os-partner:7171
TEST=chromeos-firmwareupdate --mode=recovery

Change-Id: I572be1ab28a197d3203105c91c5e00f5b646dd97
Reviewed-on: https://gerrit.chromium.org/gerrit/13461
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Puneet Kumar <puneetster@chromium.org>
Commit-Ready: Hung-Te Lin <hungte@chromium.org>