Remove never-implemented appservice-discord self-build variable

matrix_appservice_discord_container_image_self_build only switched the
image registry prefix to localhost/, but the role never had any build
tasks, so enabling it always ended in an image pull failure. Remove the
variable and reject it in validate_config.yml with a hint that the
prebuilt image is amd64-only and that mautrix-discord is the better
option on other architectures.

Fixes #2379

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

roles/custom/matrix-bridge-appservice-discord/defaults/main.yml

@@ -16,12 +16,11 @@
 # Project source code URL: https://github.com/matrix-org/matrix-appservice-discord
 
 matrix_appservice_discord_enabled: false
-matrix_appservice_discord_container_image_self_build: false
 
 # renovate: datasource=docker depName=ghcr.io/matrix-org/matrix-appservice-discord
 matrix_appservice_discord_version: v4.0.0
 matrix_appservice_discord_container_image: "{{ matrix_appservice_discord_container_image_registry_prefix }}matrix-org/matrix-appservice-discord:{{ matrix_appservice_discord_version }}"
-matrix_appservice_discord_container_image_registry_prefix: "{{ 'localhost/' if matrix_appservice_discord_container_image_self_build else matrix_appservice_discord_container_image_registry_prefix_upstream }}"
+matrix_appservice_discord_container_image_registry_prefix: "{{ matrix_appservice_discord_container_image_registry_prefix_upstream }}"
 matrix_appservice_discord_container_image_registry_prefix_upstream: "{{ matrix_appservice_discord_container_image_registry_prefix_upstream_default }}"
 matrix_appservice_discord_container_image_registry_prefix_upstream_default: "ghcr.io/"
 

roles/custom/matrix-bridge-appservice-discord/tasks/validate_config.yml

@@ -20,6 +20,7 @@
     - {'old': 'matrix_appservice_discord_docker_image_registry_prefix_upstream', 'new': 'matrix_appservice_discord_container_image_registry_prefix_upstream'}
     - {'old': 'matrix_appservice_discord_docker_image_registry_prefix_upstream_default', 'new': 'matrix_appservice_discord_container_image_registry_prefix_upstream_default'}
     - {'old': 'matrix_appservice_discord_container_image_force_pull', 'new': '<removed> (the new community.docker.docker_image_pull module handles this natively)'}
+    - {'old': 'matrix_appservice_discord_container_image_self_build', 'new': '<removed> (self-building was never actually implemented for this role, so enabling it only led to image pull failures; the prebuilt image is amd64-only, so on other architectures consider the mautrix-discord bridge instead)'}
 
 - name: Fail if required appservice-discord settings not defined
   ansible.builtin.fail: