Remove prettyprinters for JSC::Identifier and JSC::JSString.

These classes are not part of Blink (or Chromium, for that matter).

R=tony@chromium.org

Review URL: https://codereview.chromium.org/507203003

git-svn-id: svn://svn.chromium.org/blink/trunk@180982 bbb929c8-8fbe-4397-9dbb-9b2b20218538