Remove several unneeded modules from webkitpy.common.

webkitpy.common.editdistance, webkitpy.common.lru_cache, and
webkitpy.common.thread were no longer being used.

webkitpy.common.newstringio could be (and should've been) replaced by
io.StringIO() or io.BytesIO() as appropriate to indicate if you were
dealing with text or bytes.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/479633002

git-svn-id: svn://svn.chromium.org/blink/trunk@180397 bbb929c8-8fbe-4397-9dbb-9b2b20218538