Remove now-unnecessary /dev/null hacks from riot-web

`/etc/nginx/conf.d/default.conf` was previously causing
some issues when used with our `--user`.

It's not the case anymore, so we can remove it.

Fixes #369 (Github Issue).

roles/matrix-riot-web/templates/systemd/matrix-riot-web.service.j2

@@ -22,7 +22,6 @@ ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-riot-web \
 			{% endif %}
 			--tmpfs=/tmp:rw,noexec,nosuid,size=10m \
 			-v {{ matrix_riot_web_data_path }}/nginx.conf:/etc/nginx/nginx.conf:ro \
-			-v /dev/null:/etc/nginx/conf.d/default.conf:ro \
 			-v {{ matrix_riot_web_data_path }}/config.json:/app/config.json:ro \
 			-v {{ matrix_riot_web_data_path }}/config.json:/app/config.{{ matrix_server_fqn_riot }}.json:ro \
 			{% if matrix_riot_web_embedded_pages_home_path is not none %}