Remove unused sandboxing-related flags.
Remove unused sandboxing-related flags.

The original seccomp sandbox code has been removed from Chrome,
and the GPU sandbox is now enabled by default.

BUG=None
TEST=about:gpu shows sandbox is enabled.

Change-Id: I19aad15a8a43d5bfbc9f7d8982a88cc76e7bf5dc
Reviewed-on: https://gerrit.chromium.org/gerrit/57043
Tested-by: Jorge Lucangeli Obes <bc323d654c912a6c99763475eeb77a04da410a4a@chromium.org>
Reviewed-by: Antoine Labour <9b5adc77717fb53f6e082b29cd49aa0d7f0ce7f0@chromium.org>
Commit-Queue: Jorge Lucangeli Obes <bc323d654c912a6c99763475eeb77a04da410a4a@chromium.org>

chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager