go/types: remove subscripts from type parameter names

Now that we've removed the necessity for subscripts in importers, we can
effectively eliminate them from the the type parameter API by removing
them from the type string.

Change-Id: Ic4491b0dea27b0e0ce2d0636dccfaf05168ef9e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/357814


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