Cleanup after http://src.chromium.org/viewvc/blink?revision=153654&view=revision.

This is the long-standing bug where the rebaseline tool sometimes doesn't
get the right results for a platform. :(

TBR=vollick@chromium.org

git-svn-id: svn://svn.chromium.org/blink/trunk@153655 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Get rid of move_overwritten_baselines_to.

This functionality is outdated as we now always will copy baselines
as needed. No need to specially mark a bot to do so.

Also, get rid of explicitly listing is_debug in builders._exact_matches.
We can just infer that from the name of the builder.

Review URL: https://chromiumcodereview.appspot.com/18805002

git-svn-id: svn://svn.chromium.org/blink/trunk@153656 bbb929c8-8fbe-4397-9dbb-9b2b20218538