This patch removes mention of GOCACHE=off from the help/docs.
It is no longer supported in Go 1.12, per the release notes.

Fixes #29680

Change-Id: I53ab15a62743f2e55ae1d8aa50629b1bf1ae32ad
GitHub-Last-Rev: 31e904f51dece13645696a87b1164d86c984457f
GitHub-Pull-Request: golang/go#29681
Reviewed-on: https://go-review.googlesource.com/c/157517
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>