WorkerPoolImpl is only instantiated as a leaky LazyInstance, which means
its destructor is never called. GCC complains about it with
-Wunused-function, so just remove the destructor to make everything
clear.

While here, since WorkerPoolImpl::Terminate() was only called in the
destructor and in base_unittest, get rid of it as well.

R=danakj@chromium.org,dcheng@chromium.org

Review-Url: https://codereview.chromium.org/2503923002
Cr-Commit-Position: refs/heads/master@{#433506}