This commits deletes all references and code for back-bdb and back-hdb.
There is some follow up work still necessary to flush out the admin
guide for back-mdb.

In 2.4 syncrepl was changed to take its TLS defaults from the slapd
configuration (commit 1cc1f9b1) and the man page was updated to document
that (commit 27dd7a53). This updates the admin guide to match.

Update sections 6.2.2.1, 6.2.3.3

back-sql was updated in 2.4.27, back-meta in 2.4.33