cmd/compile/internal/typecheck: cleanup code

cmd/compile/internal/typecheck: cleanup code

Removing a lot of functions/variables/constants which are un-unsed
anymore in Unified IR frontend.

Change-Id: Iccf73754196bf4fa40fe701a6468f4c8a1a0c655
Reviewed-on: https://go-review.googlesource.com/c/go/+/506477
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>