Remove code from optimizer related to impl classes

WIP: https://github.com/retronym/scala/commit/77253121006602af7bacfdbc33df2e10b100e0b7

Related to #86

I'm not going to submit this as part of my nuke-impl-classes branch to keep things manageable for review. @lrytz I'll assign this one to you so you can fine tune the purge.
