Remove redundant incus_ prefix from root module variables

Rename variables to remove the useless incus_ prefix:
- incus_hypervisors → hypervisors
- incus_witnesses → witnesses
- incus_token → token
- incus_networks → networks
- incus_storage_pools → storage_pools

Updates all references across provider.tf, groups.tf, network.tf,
storage.tf, and .auto.tfvars files.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

incus/variables.tf

@@ -1,18 +1,18 @@
-variable "incus_hypervisors" {
+variable "hypervisors" {
   type = map(string)
 }
 
-variable "incus_witnesses" {
+variable "witnesses" {
   type = map(string)
 }
 
-variable "incus_token" {
+variable "token" {
   type      = string
   sensitive = true
   description = "The common authentication token for all Incus remotes"
 }
 
-variable "incus_networks" {
+variable "networks" {
   type = map(object({
     type              = string
     hypervisor_config = map(string)
@@ -20,7 +20,7 @@ variable "incus_networks" {
   }))
 }
 
-variable "incus_storage_pools" {
+variable "storage_pools" {
   type = map(object({
     driver            = string
     hypervisor_config = map(string)