Silence model in common is out of sync with Alertmanager

[alsm]

The struct definition for a Silence in common/model is out of sync with the definition in alertmanager/types, for example common/model defines the ID field as a uin64 whereas it is now a string in alertmanager/types

Perhaps as Silences are only used in Alertmanager and not Prometheus the definition should be removed from common/model?

[brian-brazil]

Yes, we should remove whatever we can from model.