update_attempter: remove unused "test mode" from ChromeOS

This unused flag is plumbed through quite a lot of code and its only
purpose is to hit a special update server hidden in the depths of
Google. This is not applicable to us, so good bye!
gpio: remove unused GPIO code from ChromeOS

The only use of this code was to the "test" flag removed in the previous
commit. This change removes update_engine's dependency on libudev.
dbus_service: remove unused method parameters

Commit f6035bf73 dropped the app_version and omaha_url arguments to the
published AttemptUpdate dbus method but were mistakenly not removed from
the function implementing AttemptUpdate. Interestingly this did not
cause problems but leaves a lot of dead code behind. Finish the job and
remove all the associated plumbing.
omaha_request: remove unused board app id setting
omaha_request: strip out channel switching logic

Under CoreOS update_engine is only ever aware of the group currently
listed in update.conf. There is no other API or other mechanism to
change the group. This means there is no distinction between
`current_channel` and `target_channel`. Additionally we do not support
downgrading so we have no use for "power washing" beforehand.
client: remove old command line args

The --omaha_url and --app_version flags have been ignored for a long
time, we can safely yank those out now.
Merge pull request #46 from marineam/force

Forced updates uber magic code removal