Fix rapid7#9160, exploit method from TcpServer

It already starts the server and waits for us. This is what was called
when the module was still auxiliary.

Author: wvu

modules/exploits/unix/http/tnftp_savefile.rb

@@ -43,11 +43,6 @@ def initialize(info = {})
     ))
   end
 
-  def exploit
-    start_service
-    sleep
-  end
-
   def on_request_uri(cli, request)
     unless request['User-Agent'] =~ /(tn|NetBSD-)ftp/
       print_status("#{request['User-Agent']} connected")