cmd/api: skip api checks

The version of cmd/api on the release-go1.4 branch
thinks the go.tools repo lives at code.google.com.
There's no point in fixing it; disable it.

Change-Id: Ieb189b1286b86b1d49a264548ee262301c95b014
Reviewed-on: https://go-review.googlesource.com/31752


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>