Remove useless variable (matrix_appservice_webhooks_control_room_id)

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1447

Seems like there's such a variable in the
`matrix-bridge-appservice-slack` role, which is probably where it got
copied over from.

roles/matrix-bridge-appservice-webhooks/defaults/main.yml

@@ -22,8 +22,6 @@ matrix_appservice_webhooks_docker_src_files_path: "{{ matrix_appservice_webhooks
 matrix_appservice_webhooks_public_endpoint: /appservice-webhooks
 matrix_appservice_webhooks_inbound_uri_prefix: "{{ matrix_homeserver_url }}{{ matrix_appservice_webhooks_public_endpoint }}"
 
-# Once you make a control room in Matrix, you can get its ID by typing any message and checking its source
-matrix_appservice_webhooks_control_room_id: ''
 matrix_appservice_webhooks_bot_name: 'webhookbot'
 matrix_appservice_webhooks_user_prefix: '_webhook'