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.h

@@ -36,7 +36,6 @@ public:
     static ErrorOr<JsonValue> from_string(StringView);
 
     JsonValue() = default;
-    explicit JsonValue(Type);
     ~JsonValue() { clear(); }
 
     JsonValue(JsonValue const&);