Help: Move resolution of help from multiple candidate IDs to HelpItem

No need for code duplication.

Change-Id: I3d2c795d072b8de5818e1844b8126e526339c0da
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/helloworld/helloworldplugin.cpp

@@ -56,7 +56,6 @@ public:
         setIcon(QIcon());
         setPriority(0);
         setId("HelloWorld.HelloWorldMode");
-        setContextHelp(QString());
     }
 };