cmd/go/internal: remove note about GOCACHE=off

Fixes golang#29680

Setting GOCACHE=off is no longer allowed in upcoming releases of Go. As described
in the Go 1.11 release notes:

> Go 1.11 will be the last release to support setting the environment variable
> `GOCACHE=off` to disable the build cache, introduced in Go 1.10. Starting in Go
> 1.12, the build cache will be required, as a step toward eliminating
> `$GOPATH/pkg`.

Setting `GOCACHE=off` now produces an error:

    build cache is disabled by GOCACHE=off, but required as of Go 1.12

This patch removes mention of `GOCACHE=off` from the help/docs.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

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.