The flag was introduced for ignition development. It can only
be used when running ignition tests in isolation on the bots.

The bots only use ignition_turbo in isolation since a while
and don't pass the --ignition flag anymore.

BUG=v8:5238

Review-Url: https://codereview.chromium.org/2197123002
Cr-Commit-Position: refs/heads/master@{#38206}