Remove API level files from `usage.md`
Remove API level files from `usage.md`

Summary: The files are now integrated into a generated module to simplify usage.

Reviewed By: Feng23

Differential Revision: D25374152

fbshipit-source-id: ea6adeb2cb4d40d36d5646331b2aef682554d2eb

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex