rapid-cdn: Drop obsolete and unusable promise

This promise was generally used to check the connection quality to the
backend with ICMP, by using ping. This is currently obsoleted by health-check
and other parameters in that group.

The parameter to setup the promise is provided by shared instance requester,
that person has no access to those results, as shared instances can't be
monitored directly, all results are only available to the cluster operator in
contrast to health-check, which results are logged to backend_log, to which
shared instance requester has access.