[RunLoop] Remove unused RunLoop::OverrideDelegateForCurrentThreadForTesting().

This is effectively a revert of r521228 while keeping the improvements
of r524097.

This was intended to be used to allow ScopedTaskEnvironment::MOCK_TIME
on top of MessageLoopForUI/ForIO but this turned out harder than
expected and we're now going with a different approach.


Bug: 708584
Change-Id: I6951ac7fc1bf6b6eac0f269e0dec9f677ea29543
Reviewed-on: https://chromium-review.googlesource.com/1066456
Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560695}
5 files changed