It would be best to diasble the cluster from the primary node first....
--you will still need to perform some cleanup steps on the second node if you want to continue using it
Here are the steps and cmdline to complete the disable ...as Mutt suggested...
Set the devices into cluster mode ( or in your case remove the cluster) with the following command and reboot the devices. Again KEYNOTE --- this is an operational mode and not a configure mode command.
> set chassis cluster cluster-id <0-15> node <0-1> reboot
For example:
SRX device x: >set chassis cluster cluster-id 1 node 0 reboot
SRX device y: >set chassis cluster cluster-id 1 node 1 reboot
1 Cluster id will be the same on both devices, but the node id should be different as one device is node0 the other device is node1
2 This command will need to be done on both devices (to setup the Cluster)
3 The range for the cluster-id is 0-15. Setting it to 0 is the equivalent of disabling cluster mode.