refactor: remove Nexus::quit() and replace with qApp->quit()

They are doing the same.

src/nexus.cpp

@@ -163,7 +163,7 @@ void Nexus::showLogin()
         Settings::getInstance().setCurrentProfile(profile->getName());
         showMainGUI();
     } else {
-        quit();
+        qApp->quit();
     }
 }
 
@@ -225,14 +225,6 @@ void Nexus::showMainGUI()
     GUI::setEnabled(true);
 }
 
-/**
- * @brief Calls QApplication::quit(), and causes Nexus::isRunning() to return false
- */
-void Nexus::quit()
-{
-    qApp->quit();
-}
-
 /**
  * @brief Returns the singleton instance.
  */

src/nexus.h

@@ -42,7 +42,6 @@ class Nexus : public QObject
 public:
     void start();
     void showMainGUI();
-    void quit();
 
     static Nexus& getInstance();
     static void destroyInstance();

src/widget/widget.cpp

@@ -589,7 +589,7 @@ void Widget::closeEvent(QCloseEvent* event)
         saveWindowGeometry();
         saveSplitterGeometry();
         QWidget::closeEvent(event);
-        Nexus::getInstance().quit();
+        qApp->quit();
     }
 }
 
@@ -638,7 +638,7 @@ void Widget::onFailedToStartCore()
         "toxcore failed to start, the application will terminate after you close this message."));
     critical.setIcon(QMessageBox::Critical);
     critical.exec();
-    Nexus::getInstance().quit();
+    qApp->exit(EXIT_FAILURE);
 }
 
 void Widget::onBadProxyCore()