devel/lab: Switch to USES=go:modules, fix build with go1.13

• Remove custom targets

> Building for lab-0.16.0

$GOPATH/go.mod exists but should not

• Error code 1

PR: 239877
Submitted by: Dmitri Goutnik <dg@syrec.org>
Approved by: swills (maintainer timeout, 15 days)