Remove redundancy between Enter and Update in 25_adding_values.html

I think there is a bit of redundancy in these two code sections?  In the `bars.enter()` chain `"x"`, `"y"`, `"width"`, `"height"`, and `"fill"` are set.  In the `bars.transition()` chain, the only things that change are `"x"` and `"width"`, so I think setting `"y"` and `"height"` there again is unnecessary.