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.

include/wx/generic/grid.h

@@ -1504,7 +1504,7 @@ public:
     void EnableCellEditControl( bool enable = true );
     void DisableCellEditControl() { EnableCellEditControl(false); }
     bool CanEnableCellControl() const;
-    bool IsCellEditControlEnabled() const;
+    bool IsCellEditControlEnabled() const { return m_cellEditCtrlEnabled; }
     bool IsCellEditControlShown() const;
 
     bool IsCurrentCellReadOnly() const;