go/types, types2: avoid unused variable error in invalid x.(type) expression

go/types, types2: avoid unused variable error in invalid x.(type) expression

This change removes one of the two follow-on errors in the issue below.

For #58612.

Change-Id: If1eec5031e524bad33caa4a914f52e6a1e273b60
Reviewed-on: https://go-review.googlesource.com/c/go/+/470015
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>