Do not restart matrix-nginx-proxy during media store import

Previously, it was more necessary to have it
(because we had a dependency between matrix-synapse and matrix-nginx-proxy)..
But nowadays, it can be removed without negative side effects.

Restarting matrix-nginx-proxy is especially bad when the proxy is not installed at all.

roles/matrix-server/tasks/import_media_store.yml

@@ -63,4 +63,3 @@
   when: stopping_result.changed
   with_items:
     - matrix-synapse
-    - matrix-nginx-proxy