[dev.regabi] cmd/compile/internal/types: remove Func.Nname

Now that there's no code remaining that uses Func.Nname, we can get
rid of it along with the remaining code that uselessly assigns to it.

Passes toolstash-check.

Change-Id: I104ab3bb5122fb824c741bc6e4d9d54fefe5646e
Reviewed-on: https://go-review.googlesource.com/c/go/+/272390


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