This removes the usage of the symfony mailer class. We did not use any of it's functionality.
We had a hard dependency on this class, but the code paths we used made it a very simple proxy. Since all usages were properly private there is no BC break.

First note the constructor of the class:

    public function __construct(TransportInterface $transport, MessageBusInterface $bus = null, EventDispatcherInterface $dispatcher = null)
    {
        $this->transport = $transport;
        $this->bus = $bus;
        $this->dispatcher = $dispatcher;
    }

We only ever called this with 1 argument, setting the transport.

Then note the only other function, which we also only ever call with 1 argument:

    public function send(RawMessage $message, Envelope $envelope = null): void
    {
        if (null === $this->bus) {
            $this->transport->send($message, $envelope);

            return;
        }
        // ... other  code omitted for brevity
   }

Since we never set the bus variable, we can skip this class altogether and directly have the transport send the message.