fix: Remove default_server listen option from fallback server

This fixes a bug introduced in commit
9b4bb07b34.

test/test_fallback.data/custom-fallback.conf

@@ -0,0 +1,5 @@
+server {
+    server_name __;
+    listen 80 default_server;
+    return 418;
+}

test/test_fallback.data/custom-fallback.yml

@@ -0,0 +1,14 @@
+services:
+  sut:
+    image: nginxproxy/nginx-proxy:test
+    volumes:
+      - /var/run/docker.sock:/tmp/docker.sock:ro
+      - ./custom-fallback.conf:/etc/nginx/conf.d/zzz-custom-fallback.conf:ro
+  http-only:
+    image: web
+    expose:
+      - "83"
+    environment:
+      WEB_PORTS: "83"
+      VIRTUAL_HOST: http-only.nginx-proxy.test
+      HTTPS_METHOD: nohttps