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>