Remove the ability to add properties to groups directly.
Remove the ability to add properties to groups directly.

We have some release group configs (zgb, x86-alex, beaglebone, etc).

When adding a new group config, users tend to assume that settings in
the release-group are shared by the configs, which is not true.

_config.add_group('beaglebone-release-group',
  _beaglebone_release.add_config('beaglebone-release',
    boards=['beaglebone'],
  ),
  _beaglebone_release.add_config('beaglebone_servo-release',
    boards=['beaglebone_servo'],
  ),

  # ANY SETTING BELOW THIS LINE IS NOT USED BY THE ABOVE CONFIGS
  tests = []
)

In truth the there is very little need to set settings directly on
the release group, so I've just removed the capability for doing so.
The only place that that functionality was used was in setting up the
x86-alex-release-group to be critical for chrome, and it wasn't actually
even needed there (we can just set up x86-alex-release to be critical,
and that'll do the job just as well.).

BUG=chromium:342520
TEST=Verify the output config is the same except the critical_for_chrome
     change to x86-alex-release
TEST=./cbuildbot_config_unittest
TEST=Try running the Chrome LKGM script and verify it still finds an LKGM

Change-Id: I5ad216eadd930684be25a31fee83ca79c5b6a725
Reviewed-on: https://chromium-review.googlesource.com/188452
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite