We will not be able to modify flags after initializing V8 (soon).
The {SingleThreadedDefaultPlatformTest} was resetting flags during
teardown for no reason, as we do not support running multiple tests in a
row anyway. Thus remove that use of {SaveFlags} and just set the
--single-threaded flag before initializing V8.

R=ahaas@chromium.org

Bug: v8:12887
Change-Id: Ia89d442cf4b2fe2e12e258da5c0c9f1f871ded12
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695378Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81024}