Remove support for old .qmlproject file format

We used to fail back to the old format if the .qmlproject file
was invalid. Anyhow, this lead to weird effects ... better disable
it explicitly.

That no warning is shown right now when loading an invalid qmlproject
file is a bug in coreplugin.

src/plugins/qmlprojectmanager/qmlproject.h

@@ -85,6 +85,8 @@ public:
     Internal::QmlProjectNode *rootProjectNode() const;
     QStringList files(FilesMode fileMode) const;
 
+    bool validProjectFile() const;
+
     enum RefreshOption {
         ProjectFile   = 0x01,
         Files         = 0x02,