Clean skippable tests that are never skipped

Author: fancyweb

Tests/Transport/SendgridApiTransportTest.php

@@ -224,10 +224,6 @@ public function testEnvelopeSenderAndRecipients()
 
     public function testTagAndMetadataHeaders()
     {
-        if (!class_exists(TagHeader::class)) {
-            $this->markTestSkipped('This test requires symfony/mailer 5.1 or higher.');
-        }
-
         $email = new Email();
         $email->getHeaders()->add(new TagHeader('category-one'));
         $email->getHeaders()->add(new MetadataHeader('Color', 'blue'));