net-mgmt/alertmanager: Switch to USES=go:modules

• Remove unneeded targets
• Fix build with go1.13beta1

> Building for alertmanager-0.16.2

go: github.com/DataDog/datadog-go@v0.0.0-20180822151419-281ae9f2d895: G
et https://proxy.golang.org/github.com/%21data%21dog/datadog-go/@v/v0.0
.0-20180822151419-281ae9f2d895.mod: dial tcp: lookup proxy.golang.org o
n 1.0.0.1:53: write udp 127.0.0.1:45569->1.0.0.1:53: write: permission
denied

• Error code 1

PR: 238917
Submitted by: Dmitri Goutnik <dg@syrec.org>
Approved by: michal@hanu.la (maintainer)