go/types, types2: handle unbound type parameters in switch (cleanup)

go/types, types2: handle unbound type parameters in switch (cleanup)

This simply moves the special handling for unbound type parameters
into the switch (which already looks for type parameters).

Change-Id: I2d6d22f3fdffc443065c3681a442288cd1d375ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/472115
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>