cmd/compile: remove unneeded type alias code in unified IR

Before #46477, the Go generics proposal allowed `type T = U` where `U`
was an uninstantiated generic type. However, we decided not to allow
that, and go/types and types2 have already been updated to disallow
it. This CL just removes the analogous code from unified IR.

Change-Id: I0fe6d1754c96790b498c1d5185b948333646d7de
Reviewed-on: https://go-review.googlesource.com/c/go/+/390315


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>