Revert "Allow customization of mixing friction & restitution."
Revert "Allow customization of mixing friction & restitution."

This reverts commit 8c9ea8d5434451c78caebae0a36054718b671529.

ruslansennov/playn,threerings/playn,threerings/playn,ruslansennov/playn,ruslansennov/playn,threerings/playn,threerings/playn