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/unix/displayx11.cpp

@@ -132,8 +132,6 @@ public:
         return IsPrimary() ? wxGetMainScreenWorkArea() : m_rect;
     }
 
-    virtual wxString GetName() const wxOVERRIDE { return wxString(); }
-
     virtual wxArrayVideoModes GetModes(const wxVideoMode& mode) const wxOVERRIDE;
     virtual wxVideoMode GetCurrentMode() const wxOVERRIDE;
     virtual bool ChangeMode(const wxVideoMode& mode) wxOVERRIDE;