Move to using `isExpressionFilter`

We can likely use `isExpressionFilter` at https://github.com/mapbox/vtshaver/blob/b0d8eb414eda21f6735228d144327e4f79b9e87d/lib/styleToFilters.js#L82-L112 to clean up that code and avoid the commented else block.

This is getting exposed at https://github.com/mapbox/mapbox-gl-js/pull/9530

Start in lib/styleToFilters.js around lines 82-112 and compare the existing logic with isExpressionFilter. Simplify that section while removing the commented else block, then verify that the resulting filter handling preserves the current behavior.

Written by the indexing model from the issue text.