Simplify the check for object keys that are themselves objects.

For object keys, we can just skip the part of `nextValue()` that parses values
that are objects or arrays. Then the existing logic for unquoted values will
already stop at `{` or `[`, and that will produce a `Missing value` exception.

src/main/java/org/json/JSONTokener.java

@@ -428,10 +428,6 @@ public class JSONTokener {
         case '"':
         case '\'':
             return this.nextString(c);
-        case '{':
-            throw syntaxError("Nested object not expected here.");
-        case '[':
-            throw syntaxError("Nested array not expected here.");
         }
 
         /*