go/printer: don't print unnecesary commas for func type param lists

Type parameter lists are not ambiguous for function declarations in the
way that they are ambiguous for type declarations. Avoid printing an
extra comma to disambiguate.

Fixes #51548

Change-Id: I8ca2b21e271982013653b9e220f92ee74f577ba2
Reviewed-on: https://go-review.googlesource.com/c/go/+/390914


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