Remove code for dealing with perf bots from garden-o-matic.

Also update the logic for which bots to hide - GPU bots should always be
hidden, everything else should be shown (except for the Perf bots, which
are being removed, of course).

R=abarth@chromium.org
BUG=294396

Review URL: https://chromiumcodereview.appspot.com/23757061

git-svn-id: svn://svn.chromium.org/blink/trunk@158045 bbb929c8-8fbe-4397-9dbb-9b2b20218538