Removing unnecessary form error margin overrides.

A discussion has started at https://github.com/zurb/foundation/commit/58fd962bc5827202d31a2a66fe8422200146cef3#L6R366, specifically line 366 of _form.scss.

The margin-bottom breaks all inputs with a parent class of .error, which would of broken docs. In 8cfb5ffec491cf0baf6b0e4114c18a00c6b22a65 a margin-top override was introduced for the sibling small element.

Instead, removing both overrides will resolve docs, and any end users.