cros_bundle_firmware: Remove test.dtb file now it is generated

This file is generated by a script, so we can remove it from the repo
now.

BUG=none
TEST=FEATURES=test sudo  -E emerge cros-devutils

Change-Id: I5071872881fe2672cce66777436c4c1bdc09a127
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/42936
Reviewed-by: Mike Frysinger <vapier@chromium.org>