[API] make DELETE "/clients/#{id}" idempotent

Not a solution, but related to https://github.com/sensu/sensu/pull/430. With mysteriously missing redis keys, `DELETE /clients/b33f` will not actually remove a client from the clients set, clear the history, or delete events. It seems a `DELETE` request ought to be idempotent and result in a clean state with or without the existence of a client key