cmd/compile/internal/types: remove ElemType wrapper

This was an artifact from when we had a separate ssa.Type interface to
break circular dependency between packages ssa and gc. It's no longer
needed now that package ssa directly uses package types.

Change-Id: I6a93e5d79082815f7f0eb89507381969cc6cb403
Reviewed-on: https://go-review.googlesource.com/109137


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>