go/types: remove qualification from type parameter strings

Type parameters only exist within the scope of a type or function
declaration, so there is really no reason to package-qualify them. It is
also confusing to do so, as it makes their type string look like a
defined type.

Fixes #49215

Change-Id: I986c527e78d45f3cadd75bc3b244e23dabe707ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/360938


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