x/tools: remove remaining files tagged for Go 1.4

(accidentally omitted from https://go-review.googlesource.com/20810)

Change-Id: Ib6ee4b2e43b6f32c5c0c031910087cc39d5c5d14
Reviewed-on: https://go-review.googlesource.com/21862
Reviewed-by: Robert Griesemer <gri@golang.org>
23 files changed