Remove apparently useless call to ShowCellEditControl()

This code seems to have been there ever since da6af900f1 (Added
drag-shrinking, 2000-02-04), but while it probably made sense back then,
it lost its purpose a long time ago as the cell editor is now not only
hidden, but also disabled when drag-resizing starts, so it doesn't make
sense to attempt showing it back when the mouse button is released: it's
never going to do anything (and didn't).

Just remove this code, as hiding the editor while drag-resizing is not
the behaviour we want from the UI point of view anyhow. And, again,
removing it doesn't actually change anything in the current version of
the code.

src/generic/grid.cpp

@@ -4663,12 +4663,6 @@ wxGrid::DoGridCellLeftUp(wxMouseEvent& event,
 
             m_waitForSlowClick = false;
         }
-        else if ( m_selection && m_selection->IsSelection() )
-        {
-            // Show the edit control, if it has been hidden for
-            // drag-shrinking.
-            ShowCellEditControl();
-        }
     }
     else if ( m_cursorMode == WXGRID_CURSOR_RESIZE_ROW )
     {