_input-group.scss & _button-group.scss still assume 1px borders

Created by: cvrebert

Refs https://github.com/twbs/bootstrap/pull/18417#issuecomment-161856850 X-Ref: #18150 (closed) Though these files no longer hardcode any border-width property values, there are some hardcoded -1px margins which were meant to counteract the border widths.