refactor(labels): Drop utility classes in favour of mixins includes

Labels were recently redesigned and the associated stylesheet was refactored to use our new way of styling components with utility mixins. However, some utility classes are still present in the current implementation. This MR fixes this by converting the following utility classes into utility mixin imports:

• .text-decoration-none
• .gl-text-black-normal
• .gl-text-white-light