feat!: remove the cluster_id variable

When creating an SKS cluster, Exoscale used to provision a cluster domain value of `<CLUSTER_ID>.cluster.local`. This is the legacy value for SKS cluster domain.

They recently transitioned to provisioning SKS cluster with the default cluster domain value of `cluster.local`.

Consequently, we need to remove the adaptations on our modules that were there to support this bizarre, and now legacy, behavior.

sks/extra-variables.tf

@@ -1,8 +1,3 @@
-variable "cluster_id" {
-  description = "ID of the SKS cluster."
-  type        = string
-}
-
 variable "metrics_storage" {
   description = "Exoscale SOS bucket configuration values for the bucket where the archived metrics will be stored."
   type = object({

sks/locals.tf

@@ -1,7 +1,6 @@
 locals {
   helm_values = [{
     thanos = {
-      clusterDomain = "${var.cluster_id}.cluster.local"
       objstoreConfig = {
         type = "S3"
         config = {