Remove apparently unnecessary wxGrid::m_inOnKeyDown

Although this variable, and a check for it in OnKeyDown(), was present
since the first version of this code added back in f85afd4e46 (Added new
wxGrid classes[...], 1999-10-06), there doesn't seem to be any
indication that it has ever been needed, so remove it to simplify the
code and make it possible to add early returns to this function easily.

No real changes yet.

include/wx/generic/grid.h

@@ -2520,7 +2520,6 @@ protected:
     wxGridCellAttr*     m_defaultCellAttr;
 
 
-    bool m_inOnKeyDown;
     int  m_batchCount;