[TechDraw] Use inherited isShow

Remove uneeded redefinitions of inherited functions

src/Mod/TechDraw/Gui/ViewProviderViewClip.cpp

@@ -87,11 +87,6 @@ void ViewProviderViewClip::hide(void)
     ViewProviderDrawingView::hide();
 }
 
-bool ViewProviderViewClip::isShow(void) const
-{
-    return Visibility.getValue();
-}
-
 bool ViewProviderViewClip::canDelete(App::DocumentObject *obj) const
 {
     // deletions of Clip objects don't destroy anything

src/Mod/TechDraw/Gui/ViewProviderViewClip.h

@@ -50,7 +50,6 @@ public:
     virtual void hide(void);
     /// Show the object in the view
     virtual void show(void);
-    virtual bool isShow(void) const;
     std::vector<App::DocumentObject*> claimChildren(void) const;
 
     virtual bool canDelete(App::DocumentObject* obj) const;