ITS#10202 slapd fails to start if compiled with --enable-overlays=yes

I am opening this MR for discussion. Running configure with --enable-overlays=yes causes nestgroup to be added twice to the slap_oinfo list, therefore slapd fails at startup. The code that I have removed here seems to be the culprit. I am not submitting this directly, since I am not sure what was the original intent, but it seems pointless - even without it, --enable-nestgroup=yes and "--enable-overlays=yes --enable-nestgroup=no" seem to work just fine. Is "mod" the issue here?