Web Animations API: Remove unused definitions that moved from Animation to TimingInput

Functionality was moved in r168575[1] but these unused definitions remained.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=168575

Review URL: https://codereview.chromium.org/227353002

git-svn-id: svn://svn.chromium.org/blink/trunk@170944 bbb929c8-8fbe-4397-9dbb-9b2b20218538