cmd/go: remove note about GOCACHE=off in docs

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>

src/cmd/go/alldocs.go

@@ -1397,7 +1397,6 @@
 // in the standard user cache directory for the current operating system.
 // Setting the GOCACHE environment variable overrides this default,
 // and running 'go env GOCACHE' prints the current cache directory.
-// You can set the variable to 'off' to disable the cache.
 //
 // The go command periodically deletes cached data that has not been
 // used recently. Running 'go clean -cache' deletes all cached data.

src/cmd/go/internal/help/helpdoc.go

@@ -705,7 +705,6 @@ The default location for cache data is a subdirectory named go-build
 in the standard user cache directory for the current operating system.
 Setting the GOCACHE environment variable overrides this default,
 and running 'go env GOCACHE' prints the current cache directory.
-You can set the variable to 'off' to disable the cache.
 
 The go command periodically deletes cached data that has not been
 used recently. Running 'go clean -cache' deletes all cached data.