Remove ability to load symbol maps

This was now mostly unused except by thread creation, which used a
symbol of the entrypoint, if available, to name the thread.

src/citra_qt/main.h

@@ -116,7 +116,6 @@ private slots:
     void OnGameListLoadFile(QString game_path);
     void OnGameListOpenSaveFolder(u64 program_id);
     void OnMenuLoadFile();
-    void OnMenuLoadSymbolMap();
     /// Called whenever a user selects the "File->Select Game List Root" menu item
     void OnMenuSelectGameListRoot();
     void OnMenuRecentFile();