Return empty string from wxDisplayImpl::GetName() by default

Simplify the code by not making this function pure virtual as all the
ports except MSW had to override it just to return an empty string.

Instead, just return empty string by default as it's not critical to
force the derived classes to override this function.

src/gtk/display.cpp

@@ -96,7 +96,6 @@ public:
     virtual wxRect GetClientArea() const wxOVERRIDE;
 
 #if wxUSE_DISPLAY
-    virtual wxString GetName() const wxOVERRIDE;
     virtual bool IsPrimary() const wxOVERRIDE;
     virtual wxArrayVideoModes GetModes(const wxVideoMode& mode) const wxOVERRIDE;
     virtual wxVideoMode GetCurrentMode() const wxOVERRIDE;
@@ -206,11 +205,6 @@ wxRect wxDisplayImplGTK::GetClientArea() const
 }
 
 #if wxUSE_DISPLAY
-wxString wxDisplayImplGTK::GetName() const
-{
-    return wxString();
-}
-
 bool wxDisplayImplGTK::IsPrimary() const
 {
 #ifdef __WXGTK4__