removed the SDB databases in contrib/sdb as they hadn't been
maintained in some time, and were no longer able to link to named
without modification. also:
- cleaned up contrib/README, which still referred to contrib
subdirectores that were removed already, and linked to an obsolete URL.
- removed references to sdb in doc/misc/roadmap and doc/misc/sdb.
4753. [contrib] Software obtainable from known upstream locations
(i.e., zkt, nslint, query-loc) has been removed.
Links to these and other packages can be found at
https://www.isc.org/community/tools [RT #46182]
