Remove `label` prop DropdownMenu's Storybook stories

[liaprins]

Describe the bug DropdownMenu's Default, Live Preview, and Test stories include a "label" option in their control panels. However, it does not change anything when you add text to this option. @tihuan suggested it is an unintentional relic from InputDropdown (which does use this prop), e.g. when DropdownMenu was changed it accidentally did not include removing the label prop. Connor has confirmed that it is not needed for the design; it also is not a prop in the MUI Menu component.