check/delete-old: Fix /bin/rmail removal condition

When WITHOUT_SENDMAIL is enabled and WITHOUT_MAILWRAPPER is disabled
we install /bin/rmail as a link to the /usr/sbin/mailwrapper.
Ensure make delete-old does not unlink /bin/rmail in that case.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D35874
MFC after: 2 weeks