These three files contain only code written for Go
(and trivial amounts at that), not any code ported
from Inferno or Plan 9.

Remove the incorrect Inferno/Plan 9 notices.

Fixes #13576.

Change-Id: Ib9901fb360232282aae5ee0f4aa527bd6f4eaaed
Reviewed-on: https://go-review.googlesource.com/17779Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>