autoupdate_EndToEnd: Remove support for MP-signed images.

This feature was long abandoned and the corresponding machinery on the
update engine deprecated. This includes:

* Removing the image_type parameter from the test and control files.

* Removing an argument from
  site_utils.autoupdate.lib.test_params.TestConfig.

* Removing MP-sign images support in the full_release_test script.

CQ-DEPEND=CL:302544
BUG=b:24404980
TEST=paygen_build_lib_unittest (with corresponding change).

Change-Id: If364fe9eff20215248022b358aaabf118362cec6
Reviewed-on: https://chromium-review.googlesource.com/302541
Commit-Ready: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
4 files changed