This removes the last remaining dual implications between sets of flags.
Support for this was originally added to support multiple subsequent
calls to {SetFlagFromString} switching a set of flags on and off. Now
that Chrome no longer relies on this behavior we can remove support for
this entirely.

Original CL: https://crrev.com/f774d8c56f00de92614886fc4cb541411eff7aa1

R=rmcilroy@chromium.org
BUG=v8:6408

Change-Id: I5f9db8457c562c0b434ea7d6eca9941c76fe7d19
Reviewed-on: https://chromium-review.googlesource.com/527174Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45782}