cmd/compile/internal/types2: reduce output when describing type parameters

There's no need to say "type parameter" for a type parameter. The
name is sufficient (they are always named), and the prose is followed
by "constrained by".

Change-Id: I98df8caa1432b8b7a874e58da6e3ed6be102b4a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/357410


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