Remove check for read only cells from IsCellEditControlEnabled()

This check was introduced back in 283b7808d8 (added support for readonly
cells and 3d border drawing, 2000-02-16), but was wrong even then and
remained wrong ever since: we must not set m_cellEditCtrlEnabled to true
when the current cell is read-only, so there is no need to check for the
latter condition if m_cellEditCtrlEnabled is indeed true.

Ensure that we really never erroneously set m_cellEditCtrlEnabled for
the read-only cells by replacing an wxASSERT_MSG checking for this in
EnableCellEditControl() with wxCHECK_RET().

Also explicitly document this function precondition, also added back in
b54ba67107 ([...] added CanEnableCellControl() and use it before calling
EnableEC, 2000-02-17) but never documented so far.

src/generic/grid.cpp

@@ -7149,12 +7149,12 @@ void wxGrid::EnableCellEditControl( bool enable )
     {
         if ( enable )
         {
+            // this should be checked by the caller!
+            wxCHECK_RET( CanEnableCellControl(), wxT("can't enable editing for this cell!") );
+
             if ( SendEvent(wxEVT_GRID_EDITOR_SHOWN) == -1 )
                 return;
 
-            // this should be checked by the caller!
-            wxASSERT_MSG( CanEnableCellControl(), wxT("can't enable editing for this cell!") );
-
             // do it before ShowCellEditControl()
             m_cellEditCtrlEnabled = enable;
 
@@ -7181,13 +7181,6 @@ bool wxGrid::CanEnableCellControl() const
         !IsCurrentCellReadOnly();
 }
 
-bool wxGrid::IsCellEditControlEnabled() const
-{
-    // the cell edit control might be disable for all cells or just for the
-    // current one if it's read only
-    return m_cellEditCtrlEnabled ? !IsCurrentCellReadOnly() : false;
-}
-
 bool wxGrid::IsCellEditControlShown() const
 {
     bool isShown = false;