Remove deprecated cluster_formation configuration

Since we bumped the minimum supported RabbitMQ version to v3.9.0 in
#1110, we can delete
the deprecated `cluster_formation.randomized_startup_delay_range`
configurations.

See rabbitmq/rabbitmq-server#3075.

Prior to this commit, the RabbitMQ logs contained the following warning:
```
2022-08-15 08:18:03.870480+00:00 [warn] <0.130.0> cluster_formation.randomized_startup_delay_range.min and cluster_formation.randomized_startup_delay_range.max are deprecated
```

Author: ansd

internal/resource/configmap.go

@@ -32,9 +32,7 @@ cluster_formation.k8s.host = kubernetes.default
 cluster_formation.k8s.address_type = hostname
 cluster_partition_handling = pause_minority
 queue_master_locator = min-masters
-disk_free_limit.absolute = 2GB
-cluster_formation.randomized_startup_delay_range.min = 0
-cluster_formation.randomized_startup_delay_range.max = 60`
+disk_free_limit.absolute = 2GB`
 
 	defaultTLSConf = `
 ssl_options.certfile = /etc/rabbitmq-tls/tls.crt

internal/resource/configmap_test.go

@@ -33,8 +33,6 @@ cluster_formation.k8s.address_type       = hostname
 cluster_partition_handling               = pause_minority
 queue_master_locator                     = min-masters
 disk_free_limit.absolute                 = 2GB
-cluster_formation.randomized_startup_delay_range.min = 0
-cluster_formation.randomized_startup_delay_range.max = 60
 cluster_name                             = ` + instanceName)
 }