[Windows Sandbox] Remove temp emergency-off switch.

Remove "WinSboxForceMsSigned" feature off switch.  The mitigation has been on for
all sandboxed child processes since M62.

R: forshaw@chromium.org
BUG: 750886
Change-Id: I9ea899e512d16db110fe9da33fa2d50758c3da82
Reviewed-on: https://chromium-review.googlesource.com/972360
Commit-Queue: Penny MacNeil <pennymac@chromium.org>
Reviewed-by: James Forshaw <forshaw@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546196}
3 files changed