JS: Tolerate trailing comma in JSON array

Previously we'd fail to extract some tsconfig.json files because of this.

javascript/extractor/src/com/semmle/js/parser/JSONParser.java

@@ -205,9 +205,6 @@ public class JSONParser {
         char c = peek();
         switch (c) {
           case ']':
-            if (!needsComma) {
-              raise("Omitted elements are not allowed in JSON.");
-            }
             next();
             break out;
           case ',':