Remove obsolete values from server.js's removeHeaders

`X-Heroku-Dynos-In-Use`, `X-Heroku-Queue-Depth` and
`X-Heroku-Queue-Wait-Time` have already been dropped in 2013:
https://devcenter.heroku.com/changelog-items/218

server.js

@@ -29,9 +29,6 @@ cors_proxy.createServer({
     'cookie',
     'cookie2',
     // Strip Heroku-specific headers
-    'x-heroku-queue-wait-time',
-    'x-heroku-queue-depth',
-    'x-heroku-dynos-in-use',
     'x-request-start',
     'x-request-id',
     'via',