Mimetypes: Remove unused API and move one function to DocumentManager

Change-Id: I97b104a75c91ce6dc4a05146b3e10c1b674f3ff0
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/coreplugin/documentmanager.h

@@ -145,6 +145,7 @@ public:
     static void setFileDialogFilter(const QString &filter);
 
     static QString fileDialogFilter(QString *selectedFilter = nullptr);
+    static QString allFilesFilterString();
 
 signals:
     /* Used to notify e.g. the code model to update the given files. Does *not*