Remove WebHeap::attachThread/detachThread

No one uses the APIs. We can add them when necessary.

This is a part of clean-up to implement a better abstraction layer for Web threads registered to Oilpan.

BUG=340522

Review URL: https://codereview.chromium.org/442133003

git-svn-id: svn://svn.chromium.org/blink/trunk@179584 bbb929c8-8fbe-4397-9dbb-9b2b20218538