Remove unused DictionaryBuilder::createString

This was added here but does not seem used:

https://codereview.chromium.org/340353006

Review URL: https://codereview.chromium.org/401443002

git-svn-id: svn://svn.chromium.org/blink/trunk@178308 bbb929c8-8fbe-4397-9dbb-9b2b20218538