cmd/compile/internal/types2: remove unused position computation (cleanup)

The position computation was needed for type list support.
With that gone, we don't need this code anymore.

Change-Id: I3f36b3d108a1fae9947fd259d4892e0287cb78ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/358701


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>