Cleanup the xmlrpc context initialization and method invocations

Apache XML-RPC offers an easy way of calling remote methods by a simple mapping in a properties file.
We've lost some of its the flexibility by hardcoding the method calls to XWikiXmlRpcApi and XWikiXmlRpcApiImpl. This was needed so that we could pass the old xwiki context to the XWikiXmlRpcApiImpl instance.
Since now we have components we could easily get the context via ExecutionContext and use the initialization made by XWikiContextInitializationFilter.