Replace wxGridCellBoolRenderer::ms_sizeCheckMark with a local var

This variable is (now) only used in a single function, so there is no
reason to declare it as a class member.

No real changes.

include/wx/generic/gridctrl.h

@@ -143,9 +143,6 @@ public:
 
     virtual wxGridCellRenderer *Clone() const wxOVERRIDE
         { return new wxGridCellBoolRenderer; }
-
-private:
-    static wxSize ms_sizeCheckMark;
 };