Remove InspectorFrontendAPI.dispatchMessageAsync

The method is unused after r171933.

BUG=364013

Review URL: https://codereview.chromium.org/242963002

git-svn-id: svn://svn.chromium.org/blink/trunk@171942 bbb929c8-8fbe-4397-9dbb-9b2b20218538