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

[dchagin]

Summary

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.

[emaste]

corresponding case in usr.sbin/mailwrapper/Makefile

.if ${MK_MAILWRAPPER} != "no" && ${MK_SENDMAIL} == "no"
SYMLINKS+=      ..${BINDIR}/mailwrapper /bin/rmail
.endif