Remove unused base::Thread::thread_handle() getter.

Fix only user (dbus/test_server.cc) to use Thread on stack which is
automatically joined on destruction (and thus doesn't need the exposed
handle it was using to manually join).

BUG=629716

Review-Url: https://codereview.chromium.org/2162293002
Cr-Commit-Position: refs/heads/master@{#406831}
2 files changed