remove URL rewriting in the nginx proxy

Removes the URL rewriting in the nginx proxy.  This is no longer needed after the SlipStreamServer has been patched to consistently use the URL scheme passed by the proxy.

(See SlipStream/SlipStreamServer#177 and SlipStream/SlipStreamServer#180)
@schaubl explained that it's line 18 that retrieves the http/https value (a nginx specific variable).