Retry alertmanager delete silence when one instance fails

[Emmanuel Lodovice]

Describe the bug
Currently alertmanager silence handler only retries on GET requests https://github.com/cortexproject/cortex/blob/master/pkg/alertmanager/distributor.go#L250-L260 but it seems like from the discussion in #4422 we should also be able to retry on DELETE.

Expected behavior
Delete silence request should retry to a difference instance if the delete request in one AM instance fails.