Remove AnimatableValue::add

This is currently unused and we probably won't use the AnimatableValue
machinery when we end up implementing add composition in the future.

BUG=257591

Review URL: https://codereview.chromium.org/235983013

git-svn-id: svn://svn.chromium.org/blink/trunk@171560 bbb929c8-8fbe-4397-9dbb-9b2b20218538