AK+LibJS: Remove error-prone JsonValue constructor

Consider the following:

    JsonValue value { JsonValue::Type::Object };
    value.as_object().set("foo"sv, "bar"sv);

The JsonValue(Type) constructor does not initialize the underlying union
that stores its value. Thus JsonValue::as_object() will A) refer to an
uninitialized union member, B) deference that member.

This constructor only has 2 users, both of which initialize the type to
Type::Null. Rather than implementing unused functionality here, replace
those uses with the default JsonValue constructor, and remove the faulty
constructor.

AK/JsonParser.cpp

@@ -304,7 +304,7 @@ ErrorOr<JsonValue> JsonParser::parse_null()
 {
     if (!consume_specific("null"))
         return Error::from_string_literal("JsonParser: Expected 'null'");
-    return JsonValue(JsonValue::Type::Null);
+    return JsonValue {};
 }
 
 ErrorOr<JsonValue> JsonParser::parse_helper()