In the ${MK_SENDMAIL} == no case, /etc/rc.sendmail should be removed as well as it is sendmail specific. OLD_FILES+=etc/rc.sendmail
This is in line with etc/Makefile which doesn't install rc.sendmail in the ${MK_SENDMAIL} == no case.
I just noticed that, likewise, rc.d/sendmail should be removed as well. It's not installed in the ${MK_SENDMAIL} == no case either, and mergemaster offers to delete it anyhow.