Optimize uninstall tasks a bit

- forego removing Docker images - it's not effective anyway, because it
  only removes the last version.. which is a drop in the bucket, usually

- do not reload systemd - it's none of our business. `--tags=start`,
  etc., handle this

- combine all uninstall tasks under a single block, which only runs if
  we detect traces (a leftover systemd .service file) of the component.
  If no such .service is detected, we skip them all. This may lead to
  incorect cleanup in rare cases, but is good enough for the most part.

roles/custom/matrix-nginx-proxy/tasks/setup_nginx_proxy.yml

@@ -239,7 +239,6 @@
     state: stopped
     enabled: false
     daemon_reload: true
-  register: stopping_result
   when: "not matrix_nginx_proxy_enabled | bool and matrix_nginx_proxy_service_stat.stat.exists"
 
 - name: Ensure matrix-nginx-proxy.service doesn't exist