Make gpg_secret_backend mount-only; plugin import split out

Plugin catalog registration now lives in its own module + config/plugins (see
the split-out PR), so this module no longer registers the plugin.

- Drop the vault_plugin resource and the sha256/command inputs; the mount now
  references the already-registered plugin type directly.
- Simplify config/gpg_secret_backend/gpg.yaml to just the mount description.

modules/vault_cluster/main.tf

@@ -341,8 +341,6 @@ module "gpg_secret_backend" {
 
   path        = each.key
   plugin      = each.value.plugin
-  command     = each.value.command
-  sha256      = each.value.sha256
   description = each.value.description
 }