cmd/compile: deprecate has init and derived func instance

Removes 'has init' and 'derived func instance' fields from unified IR
starting with V2.

This should be a no-op at the moment as the writer is hardwired to create V1.

Updates #68778

Change-Id: I84a606cbc27cd6d8bd6eee2aff44c89f4aa7413c
Reviewed-on: https://go-review.googlesource.com/c/go/+/606035


Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>