Removing it from default variants set which will substantially increase test speed for developers.

Pure ignition is intended to be tested only on bots unless opted in.

BUG=chromium:682617
R=mstarzinger@chromium.org,mvstanton@chromium.org,rmcilroy@chromium.org

Change-Id: I98adb81951a0a2d16f52e6b34b98d3a90e178d8a
Reviewed-on: https://chromium-review.googlesource.com/452580Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43721}