refactor(css): Remove dropdown-specific mixins

This MR replaces mixins that declare dimension properties of dropdowns (width, height, etc.) with SASS variables. We can use variables to declare these custom dimensions, and use those variables in the component’s styles. This decision does not follow the guideline of using utility mixins to style components. This is a valid exception because the dropdown specs define dimensions that are too specific to be encapsulated in a generic utility class. Moreover, mixins declared in scss/mixins should have a broader purpose.