Removed the "Join Lines Up" action

It was a bit confusing, since it reversed the order of the lines. When
this would be fixed, the added value of this action is very small, since
you can then also simply move your cursor one up before joining the
lines.

src/plugins/texteditor/basetexteditor.h

@@ -412,8 +412,7 @@ public slots:
     void copyLineUp();
     void copyLineDown();
 
-    void joinLineUp();
-    void joinLineDown();
+    void joinLines();
 
     void cleanWhitespace();
 
@@ -623,7 +622,6 @@ private:
     void handleBackspaceKey();
     void moveLineUpDown(bool up);
     void copyLineUpDown(bool up);
-    void joinLineUpDown(bool up);
     void saveCurrentCursorPositionForNavigation();
     void updateCurrentLineHighlight();