fpspreadsheet: Fix formulas not being recalculated when a cell value is deleted in the grid by DEL key (https://forum.lazarus.freepascal.org/index.php/topic,42365.0.html)

git-svn-id: https://svn.code.sf.net/p/lazarus-ccr/svn@6615 8e941d3f-bd1b-0410-a28a-d453659cc2b4

components/fpspreadsheet/source/common/fpspreadsheet.pas

@@ -2239,6 +2239,8 @@ begin
     else
       // Erase all cell content
       InitCell(nil, r, c, ACell^);
+
+    ChangedCell(r, c);
   end;
 end;