Removed unused Proxy plugin

The ProxyPlugin was never called from anywhere in the core code. I also cannot figure what it used to do. Instruct plugins that a proxy is being used? Configure Crawljax?

I don't see the use of it now so I cleaned it up. Configuring the proxy is done via the CrawljaxConfigurationBuilder. Plugins don't need to know a proxy server is being used as far as I know.