internal/apidiff: remove

This was added in preparation for gorelease, which did not progress.

Change-Id: Ieaf93feddcb3bcbf0bd191cad7941786f8e544b3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/647695
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: jba