newrelic: fix newrelic starting without license key.

Fix an issue where the New Relic agent always started in the production
environment, even when a license key wasn't configured.

Also make the New Relic agent log to stdout instead of log/newrelic_agent.log.

lib/capistrano/tasks/symlink.rake

@@ -3,9 +3,6 @@ namespace :symlink do
   task :local_files do
     on roles(:app, :worker) do
       execute :ln, "-s", "#{deploy_to}/shared/config/danbooru_local_config.rb", "#{release_path}/config/danbooru_local_config.rb"
-      if test("[ -f #{deploy_to}/shared/config/newrelic.yml ]")
-        execute :ln, "-s", "#{deploy_to}/shared/config/newrelic.yml", "#{release_path}/config/newrelic.yml"
-      end
     end
   end