cmd/compile/internal/types2: remove (*Signature).SetRecvTypeParams

This method isn't available in go/types, and its use by unified IR is
non-essential. This CL refactors reader2.go to avoid using it and then
removes the method.

Change-Id: I813c93a062c43292bb6760686ef91df5219534a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/380834


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>