rem: usr: Remove RBTDB implementation

Remove the RBTDB database implementation, and only leave the QPDB
based implementations of zone and cache databases.  This means
it's no longer possible to choose the RBTDB to be default at the
compilation time and it's not possible to configure RBTDB as the
database backend in the configuration file.

Closes #5027

Merge branch '5027-remove-rbtdb-implementation' into 'main'

See merge request isc-projects/bind9!9733