v4: label-pill doesn't adhere to $enable-rounded

Created by: davidjb

See https://github.com/twbs/bootstrap/blob/v4-dev/scss/_labels.scss#L46

This makes .label-pill inconsistent with .nav-pills; the latter uses $enable-rounded via the border-radius mixin and the former hard-codes its value.

That said, if .label-pill adhered to this variable, there'd be no difference between standard .label and .label-pill if $enabled-rounded was disabled, thus making it redundant.