Core/Help: Clean up HelpManager interface and users

Change-Id: Ia6edf583f2a002d7c6f4878df670a78614ea087d
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/coreplugin/coreplugin.cpp

@@ -114,7 +114,7 @@ void CorePlugin::extensionsInitialized()
 
 bool CorePlugin::delayedInitialize()
 {
-    HelpManager::instance()->setupHelpManager();
+    HelpManager::setupHelpManager();
     return true;
 }