Removes unused variable from customer sh module

* Wasn't used
* Was the wrong way even if we wanted to do that

base/splunk_servers/customer_searchhead/vars.tf

@@ -1,8 +1,3 @@
-variable "splunk_customer_cidrs" {
-  description = "List of customer's cidrs that can access the SH"
-  type = list(string)
-}
-
 variable "instance_name" {
   description = "[Optional] Override the Instance Name"
   type = string