fix: remove unused argocd_hostname variable

Dead code leftover from the Gateway API migration — never wired into any
templatefile or resource. Removes tflint terraform_unused_declarations warning.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mbologna

vars.tf

@@ -311,12 +311,6 @@ variable "certmanager_email_address" {
 
 # ── ArgoCD (always installed — GitOps controller keeps cluster active) ────────
 
-variable "argocd_hostname" {
-  type        = string
-  description = "Fully-qualified hostname for the ArgoCD UI (e.g. argocd.example.com). When set, a Gateway API HTTPRoute with a cert-manager TLS certificate is created."
-  default     = null
-}
-
 variable "longhorn_hostname" {
   type        = string
   description = "Fully-qualified hostname for the Longhorn UI (e.g. longhorn.example.com). When set, a Gateway API HTTPRoute with BasicAuth (Envoy Gateway SecurityPolicy) and a cert-manager TLS certificate is created."