Web Animations: Remove confusing interpolation subdirectory.

The current distribution of files doesn't make sense. I have an idea for a
better split that I'd like to try, but that's speculative, so first we should
reset to a good state.

Review URL: https://codereview.chromium.org/408443004

git-svn-id: svn://svn.chromium.org/blink/trunk@178534 bbb929c8-8fbe-4397-9dbb-9b2b20218538