AK: Allow destruction of JsonObjectSerializer objects after errors

Previously we'd VERIFY() that the user had called finish(). This makes
the following code incorrect though:

auto json = TRY(JsonObjectSerializer<>::try_create(builder));
TRY(json.add("total_time"sv, total_time_scheduled.total));
TRY(json.finish());
return ...;

If the second TRY() returns early we'd fail at the VERIFY() call in the
destructor.

Calling finish() in the destructor - like we had done earlier - is also
not helpful because we have no idea whether the builder is still valid.
Plus we wouldn't be able to handle any errors for that call.

Verifying that either finish() was called or an error occurred doesn't
work either because the caller might have multiple Json*Serializer
objects, e.g. when inserting a JSON array into a JSON object. Forcing
the user to call finish() on their "main" object when a sub-object
caused an error seems unnecessarily tedious.

AK/JsonArraySerializer.h

@@ -43,11 +43,6 @@ class JsonArraySerializer {
 
  JsonArraySerializer(JsonArraySerializer const&) = delete;
 
- ~JsonArraySerializer()
- {
- VERIFY(m_finished);
- }
-
 #ifndef KERNEL
  ErrorOr<void> add(JsonValue const& value)
  {

AK/JsonObjectSerializer.h

@@ -38,11 +38,6 @@ class JsonObjectSerializer {
 
  JsonObjectSerializer(JsonObjectSerializer const&) = delete;
 
- ~JsonObjectSerializer()
- {
- VERIFY(m_finished);
- }
-
 #ifndef KERNEL
  ErrorOr<void> add(StringView key, JsonValue const& value)
  {