Remove Mandrill mail driver

This is in preparation for the upcoming upgrade to Laravel 6,
which dropped this driver.

Refs #2055.

src/Mail/MailServiceProvider.php

@@ -24,7 +24,6 @@ class MailServiceProvider extends AbstractServiceProvider
             return [
                 'mail' => SendmailDriver::class,
                 'mailgun' => MailgunDriver::class,
-                'mandrill' => MandrillDriver::class,
                 'log' => LogDriver::class,
                 'smtp' => SmtpDriver::class,
             ];

src/Mail/MandrillDriver.php

@@ -1,47 +0,0 @@
-<?php
-
-/*
- * This file is part of Flarum.
- *
- * For detailed copyright and license information, please view the
- * LICENSE file that was distributed with this source code.
- */
-
-namespace Flarum\Mail;
-
-use Flarum\Settings\SettingsRepositoryInterface;
-use GuzzleHttp\Client;
-use Illuminate\Contracts\Validation\Factory;
-use Illuminate\Mail\Transport\MandrillTransport;
-use Illuminate\Support\MessageBag;
-use Swift_Transport;
-
-class MandrillDriver implements DriverInterface
-{
-    public function availableSettings(): array
-    {
-        return [
-            'mail_mandrill_secret' => '',
-        ];
-    }
-
-    public function validate(SettingsRepositoryInterface $settings, Factory $validator): MessageBag
-    {
-        return $validator->make($settings->all(), [
-            'mail_mandrill_secret' => 'required',
-        ])->errors();
-    }
-
-    public function canSend(): bool
-    {
-        return true;
-    }
-
-    public function buildTransport(SettingsRepositoryInterface $settings): Swift_Transport
-    {
-        return new MandrillTransport(
-            new Client(['connect_timeout' => 60]),
-            $settings->get('mail_mandrill_secret')
-        );
-    }
-}