Remove global .start, .end, .text-start, and .text-end classes

Commit a13a0db7b4379f0cacbb3cf30f63fc1ee20d03e5 added four new global classes intended to be used for bidirectionally. This broke the `.end` class in _grid.scss.

These new classes haven't been documented anywhere and were added in an unrelated pull request, https://github.com/zurb/foundation/pull/4035. From what I can tell, the intention is that you can use `.start` instead of `.left` to float an element to either the left or the right depending on the document's text direction. Likewise for the other added classes. I guess this has some semantic value, but I think it dangerously pollutes the global namespace, especially since these use `!important`.

We can simply continue using `.left` `.right` `.text-left` and `.text-right` depending on what side we want something. I don't see the use-case for abstracting this to `.start` `.end` `.text-start` and `.text-end`.

This closes #4175.