Remove unused invalid Swiftmailer file

[MAILPOET-2442]

prefixer/fix-swiftmailer.php

@@ -27,3 +27,6 @@ foreach ($files as $file) {
     file_put_contents($file, $data);
   }
 }
+
+# remove unused PHP file that starts with shebang line instead of <?php and causes PHP lint on WP repo to fail
+exec('rm -r ' . __DIR__ . '/../vendor-prefixed/swiftmailer/swiftmailer/lib/swiftmailer_generate_mimes_config.php');