FCPPluginClient: Compile-fix imports
Delete interface FredPluginFCPServer:
- Fix users to use the replacement ServerSideFCPMessageHandler. Done with Eclipse auto-renaming and then manually reviewed: In some instances, it made sense to explicitely reference it as "FredPluginFCPMessageHandler.ServerSideFCPMessageHandler" instead of only "ServerSideFCPMessageHandler" because we want plugins to use interfaces which start with "FredPlugin*". So in the classes relevant to them, we should stress that by referencing the extended name.
- Removes the FIXME demanding that at FredPluginFCPMessageHandler, also corrects the FIXME at ClientSideFCPMessageHandler which should have referenced the FredPluginFCPClient, not *Server