Thread entity stored 4 times in PrivateMessageForm

Problem/Motivation

We can get the thread entity in 4 different ways:
- $entity = $formState->get('pmt_entity');
- $private_message_thread = $formState->get('thread');
- $formState->set('private_message_thread', $private_message_thread);
- $this->entity

Proposed resolution

- refactored the code to have only one entity object inside PrivateMessageForm and its $formState.