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

[Debian testing autoremoval watch]

prometheus-alertmanager 0.15.3+ds-2 is marked for autoremoval from testing on 2019-03-31

It (build-)depends on packages with these RC bugs:
922389: golang-protobuf-extensions: FTBFS (cannot find package "github.com/golang/protobuf/proto/testdata")