Revert "cassandra: Update the rack information of the 2 new cassandra servers"

This reverts commit 32116116.

We'll not use the rack affinity because our server dispatching
in the rack is not compatible with the cassandra needs.
It will also add some constraints on the cluster provisionning
we could not afford (need to keep the same number of servers in
all the racks)

Related to swh/infra/sysadm-environment#4801

data/common/cassandra.yaml

@@ -39,12 +39,10 @@ cassandra::nodes:
     instances:
       instance1:
         cluster_name: archive_production
-        rack: 30C11
   cassandra08.internal.softwareheritage.org:
     instances:
       instance1:
         cluster_name: archive_production
-        rack: 30C11
 
 cassandra::clusters:
   archive_production: