Remove Unicorn.

No longer used now that we use Puma in production. If you still used
Unicorn in your install, switch to `bin/rails server` instead. See
config/puma.rb for config settings.

script/install/nginx.danbooru.conf

@@ -53,6 +53,5 @@ server {
 }
 
 upstream app_server {
-  # server 127.0.0.1:9000;
-  server unix:/tmp/.unicorn.sock;
+  server 127.0.0.1:3000;
 }