DevTools: Remove InspectorFrontendAPIImpl.prototype.loadCompleted()

https://crrev.com/6bffde4d87f7a05e5a871cd48366d56b8b83e950 introduces
dispatching of messages on InspectorFrontendAPI post the
IFH.loadCompleted() call only, hence the IFAPI.loadCompleted() call
is no longer required.

R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/644493003

git-svn-id: svn://svn.chromium.org/blink/trunk@183601 bbb929c8-8fbe-4397-9dbb-9b2b20218538