Retry alertmanager delete silence when one instance fails

[emanlodovice]

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 https://github.com/cortexproject/cortex/pull/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.