Contribution page does not respect "Contribution Amounts Label" when page is also used for memberships

"Contribution Amounts Label" is a required field on the Amounts tab of a contribution config page. If the membership section is enabled, the user-entered label is ignored and "Additional Contribution" is used instead.

If we require the user to provide a value, we should respect that value.

Changing the behavior may surprise some people; e.g., if they have nonsense in that field and are relying on the hardcoded label.