Is netbackup installed as cluster-aware. This means, is netbackup configured to failover?
IF netbackup is not installed cluster-aware, then you can safely remove the PBX-ClusterGroup.
Refer this:
http://seer.entsupport.symantec.com/docs/287349.htm
I believe that even if you have NOM, it should be safe to remove the service group as long as the PBX service is set to "automatic" in services.msc
NOTE: Apart from Netbackup, PBX is also used by Storage Foundation products. (For VEA)