Issue #3528939: Remove swiftmailer migration code

Closes #3528939

modules/mailer_policy/config/install/mailer_policy.mailer_policy._.yml

@@ -7,6 +7,5 @@ configuration:
   mailer_inline_css: {}
   mailer_wrap_and_convert:
     plain: false
-    swiftmailer: false
   email_theme:
     theme: _active_fallback