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/JsonValue.cpp

@@ -15,11 +15,6 @@
 
 namespace AK {
 
-JsonValue::JsonValue(Type type)
-    : m_type(type)
-{
-}
-
 JsonValue::JsonValue(JsonValue const& other)
 {
     copy_from(other);