Removes the --ignition-staging flag since it is no longer used
by anything and won't be a shipping configuration. Also removes
ignition_turbo variant from testrunner, since it is now 
the same as the turbofan variant.

BUG=v8:4280

Change-Id: I3b96e986879fc70b8e202fe9496334828acdd0ba
Reviewed-on: https://chromium-review.googlesource.com/452621
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43806}