[9.18] rem: usr: Move contributed DLZ modules into a separate repository

The DLZ modules are poorly maintained as we only ensure they can still
be compiled, the DLZ interface is blocking, so anything that blocks the
query to the database blocks the whole server and they should not be
used except in testing.  The DLZ interface itself is going to be scheduled
for removal.

The DLZ modules now live in https://gitlab.isc.org/isc-projects/dlz-modules
repository.

Closes #4865

Backport of MR !9349

Merge branch 'backport-4865-remove-contributed-DLZ-modules-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9778