[css-grid] Fix crash removing positioned grid item
When we add or remove a positioned item we don't need to mark
the grid as dirty, because positioned items do not affect the layout
of the grid at all.
This was causing a crash when a positioned item was removed
after a layout. As after the positioned item was removed,
the method LayoutGrid::layoutBlock() was not called,
so when the grid was repainted we got a crash.