[base] Drop obsolete Thread::YieldCPU.

The method is not used anywhere, and it is a bad idea in general anyway.
If you see a need to call YieldCPU, then you're code is probably in need
of a redesign!

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/1116853002

Cr-Commit-Position: refs/heads/master@{#28147}