This removes the explicit --write-protect-code-memory flag from the list
of flags in the "stress_incremental_marking". The feature is enabled by
default by now and no longer needs explicit testing.

R=ulan@chromium.org
BUG=v8:6792

Change-Id: I5d6ba21dff261488bbe1b0148ce204bf78d57334
Reviewed-on: https://chromium-review.googlesource.com/860661Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50500}