Update step.less

This will fix issue #699 sub issue 1) (see: https://github.com/Semantic-Org/Semantic-UI/issues/699)

This will remove rounded corners in the wrong locations of a vertical step including removing rounded corners when a first/last child is active (the rounded corners do not let the arrow lay flush with the step).
Merge pull request #725 from msj121/master

Update step.less