www/grafana6: Switch to USES=go:modules, fix build with go1.13
> Building for grafana6-6.3.3_1
$GOPATH/go.mod exists but should not
- Error code 1
- Remove custom build and install targets
PR: 240317
Submitted by: Dmitri Goutnik <dg@syrec.org>
Pointy hat: jlaffaye