Removed Central proxy option

With the Nexus analyzer, it's necessary to determine whether a proxy is necessary as both the old central search and internal repositories used the same interface. With Central search, only Central (AFAIK) has this option, and so the global proxy configuration should suffice. This removes the option for whether to use the proxy with Central - if it's configured globally, it'll be used.

Also, renamed every reference of Solr (the actual search engine method being used) to Central (the actual implementation of Solr being used.)