cmd/compile/internal/typecheck: prune some useless code

cmd/compile/internal/typecheck: prune some useless code

This error checking code is all obsolete by types2.

Change-Id: I247cee2c847236dfbd5a878441ad712481692927
Reviewed-on: https://go-review.googlesource.com/c/go/+/520607
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>