chg: dev: Don't retain the default configuration tree

The built-in configuration is actually used in two cases: first, when the server is loaded (or reloaded), and second when `rndc showconf -builtin` is called.

Considering the parsing of the builtin configuration is quick and does not occur during exclusive mode, but the configuration tree takes considerable memory space, the built-in configuration is no longer kept in memory once it has been used; instead it is re-parsed on demand.

Merge branch 'colin/not-persist-builtin-config' into 'main'

See merge request isc-projects/bind9!11187