[pkg-go] prometheus-alertmanager is marked for autoremoval from testing

[Debian testing autoremoval watch]

prometheus-alertmanager 0.20.0+ds-1 is marked for autoremoval from testing on 2020-02-16

It (build-)depends on packages with these RC bugs:
947996: golang-github-mailru-easyjson: golang-github-mailru-easyjson FTBFS in testing/unstable