Revert "allow type parameter instantiation in cases branches"

This reverts commit 934364c9c86f6202a8d63a5791613c6547bb76b8.

This is unnecessarily explicit, as the type parameters from the cases() position can be used. Doesn't seem to warrant inclusion in the grammar.
Merge branch 'app-params' of github.com:brownplt/pyret-lang into app-params