Remove x-webkit-speech implementation

The API has been considered deprecated for some time, was disabled in
https://src.chromium.org/viewvc/blink?revision=171373&view=revision

BUG=223198

Review URL: https://codereview.chromium.org/268863004

git-svn-id: svn://svn.chromium.org/blink/trunk@173329 bbb929c8-8fbe-4397-9dbb-9b2b20218538