DevTools: remove unused private methods from front-end

This patch removes unused methods from front-end. The methods were
listed with the following script: https://gist.github.com/aslushnikov/9208534

BUG=

Review URL: https://codereview.chromium.org/179863002

git-svn-id: svn://svn.chromium.org/blink/trunk@167797 bbb929c8-8fbe-4397-9dbb-9b2b20218538