Cleanup SplashScreen class

Cleaning up after replacing the QSplashScreen base class with the
QWidget class.

src/qt/splashscreen.h

@@ -5,8 +5,7 @@
 #ifndef BITCOIN_QT_SPLASHSCREEN_H
 #define BITCOIN_QT_SPLASHSCREEN_H
 
-#include <functional>
-#include <QSplashScreen>
+#include <QWidget>
 
 #include <memory>
 
@@ -37,8 +36,8 @@ protected:
     void closeEvent(QCloseEvent *event);
 
 public Q_SLOTS:
-    /** Slot to call finish() method as it's not defined as slot */
-    void slotFinish(QWidget *mainWin);
+    /** Hide the splash screen window and schedule the splash screen object for deletion */
+    void finish();
 
     /** Show message and progress */
     void showMessage(const QString &message, int alignment, const QColor &color);