Move to using `isExpressionFilter`

[springmeyer]

We can likely use isExpressionFilter at

vtshaver/lib/styleToFilters.js

Lines 82 to 112 in b0d8eb4

	function getPropertyFromFilter(filter, properties) {
	if (styleSpec.expression.isExpression(filter)) {
	getPropertyFromExpression(filter, properties);
	}
	// Warning: Below code should put in to an else conditions,
	// but since the `isExpression` can not tell it is a expression or filter syntax I put it outsied the else
	// this could reduce the performance or cause some potential bugs, we must keep an eye on this.
	// else {
	let subFilter = [];
	for (let i = 0; i < filter.length; i++) {
	if (typeof filter[i] === 'object' && filter[i] instanceof Array) {
	subFilter.push(filter[i]);
	}
	}
	if (subFilter.length > 0) {
	subFilter.forEach(sfilter => {
	getPropertyFromFilter(sfilter, properties);
	})
	} else {
	if (filter.length >= 3 && typeof filter[1] === 'string') {
	if (filter[1].indexOf('$') === -1) {
	properties.push(filter[1]);
	}
	}
	}
	// }
	}

to clean up that code and avoid the commented else block.

This is getting exposed at mapbox/mapbox-gl-js#9530