databases/pgmetrics: Switch to USES=go:modules

• Remove unneeded targets
• Drop nop BUILD_AS_NON_ROOT while here
• Fix build with go1.13beta1

> Building for pgmetrics-1.7.0

$GOPATH/go.mod exists but should not

• Error code 1

PR: 238930
Submitted by: Dmitri Goutnik <dg@syrec.org>
Approved by: girgen (maintainer timeout, 24 days)