WIP towards #5446

This removes the unneeded 4of6 unit and moves the guttered versions to elgg-grid-gutters.

I may have made a mistake, but from the grid demo it doesn't look like the guttered versions work. The last column is not floated, so the margin is placed before the first column.

I don't see how the gutters can work like they're designed. Consider one row with 1/5 + 4/5 and another row with 1/5 + 2/5 + 2/5. Will these have the same gutters? I'd think the gutters would need to be applied evenly inside the column elements.

I've never had good luck with grids with gutters, mainly because nesting makes the gutters differ in width.