refactor: remove closeWindow because no one need it

app/mainwindow.cpp

@@ -345,10 +345,6 @@ void MainWindow::centerWindow()
     );
 }
 
-void MainWindow::closeWindow()
-{
-}
-
 void MainWindow::toggleAvoidResetTransform()
 {
     ui->m_gv->setAvoidResetTransform(!ui->m_gv->isAvoidResetTransform());

app/mainwindow.h

@@ -44,7 +44,6 @@ protected slots:
     void dropEvent(QDropEvent *event) override;
 
     void centerWindow();
-    void closeWindow();
     void toggleAvoidResetTransform();
     void toggleFullscreen();
     void toggleMaximize();