Prevent unnecessary CSS output for emphasized links

Created by: ysds

If $emphasized-link-hover-darken-percentage is 0 or 0%, the overwrite for a is not necessary:

.text-primary {
  color: #0d67cb !important;
}

a.text-primary:hover, a.text-primary:focus {
  color: #0d67cb !important; // No need
}

This PR prevents unnecessary CSS output when $emphasized-link-hover-darken-percentage is 0 or 0%