Remove layout tests for the legacy x-webkit-speech API

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

Removing the tests allows us to remove Chromium's MockWebSpeechInputController,
which is the last consumer of the public WebSpeechInput* classes.

BUG=223198

Review URL: https://codereview.chromium.org/264973003

git-svn-id: svn://svn.chromium.org/blink/trunk@173128 bbb929c8-8fbe-4397-9dbb-9b2b20218538