Remove unused literal value serialization of Collator.

We no longer constant-fold Collator expressions, so there's no way to create a literal holding a Collator.

src/style-spec/expression/definitions/collator.js

@@ -65,16 +65,6 @@ export class Collator {
         return new Intl.Collator(this.locale ? this.locale : [])
             .resolvedOptions().locale;
     }
-
-    serialize() {
-        const options = {};
-        options['caseSensitive'] = this.sensitivity === 'variant' || this.sensitivity === 'case';
-        options['diacriticSensitive'] = this.sensitivity === 'variant' || this.sensitivity === 'accent';
-        if (this.locale) {
-            options['locale'] = this.locale;
-        }
-        return ["collator", options];
-    }
 }
 
 export class CollatorExpression implements Expression {

src/style-spec/expression/definitions/literal.js

@@ -60,10 +60,6 @@ class Literal implements Expression {
             // couldn't actually generate with a "literal" expression,
             // so we have to implement an equivalent serialization here
             return ["rgba"].concat(this.value.toArray());
-        } else if (this.value instanceof Collator) {
-            // Same as Color above: literal serialization delegated to
-            // Collator (not CollatorExpression)
-            return this.value.serialize();
         } else {
             assert(this.value === null ||
                 typeof this.value === 'string' ||