mgr/dashboard: prevent alert redirect

mgr/dashboard: prevent alert redirect

Prevent Alertmanager alerts from being redirected to the active mgr
dashboard instance. There are two reasons for it:

1. It doesn't bring any additional benefit. The Alertmanager config
   includes all available mgr instances - active and passive ones. In
   case of an alert, it will be sent to all of them. It ensures that
   the active mgr dashboard will receive the alert in any case.
2. The redirect URL includes the mgr IP and NOT the FQDN. This leads
   to issues in environments where an SSL certificate is configured and
   matches the FQDNs, only.

Fixes: https://tracker.ceph.com/issues/56401
Signed-off-by: Tatjana Dehler <tdehler@suse.com>