go/types, types2: remove parse (we only need mustParse for tests)

go/types, types2: remove parse (we only need mustParse for tests)

While at it, also simplify mustTypecheck again as it can just use
typecheck.

Change-Id: I6cb07b1078d9a39e0f22851028fdd4442127f2f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/490015
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>