Remove dead code and move private APIs to bottom of MotionRuntime.
Remove dead code and move private APIs to bottom of MotionRuntime.

Reviewers: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, chuga

Reviewed By: O4 Material Apple platform reviewers, chuga

Tags: #material_motion

Differential Revision: http://codereview.cc/D2746

material-motion/material-motion-swift,material-motion/material-motion-swift