[5.4] Removes unwanted dependency of illuminate/mail

[nunomaduro]

On the MailServiceProvider the method registerMarkdownRenderer is using the helper function config() on the following code:

        $this->app->singleton(Markdown::class, function () {
            return new Markdown($this->app->make('view'), [
                'theme' => config('mail.markdown.theme', 'default'),
                'paths' => config('mail.markdown.paths', []),
            ]);
        });

Problem: On the composer.json of the package illuminate/mail the package illuminate/foundation don't exists. So the helper function config() also don't exists.

Solucion: Makes usage of the app container to get the markdown app config.