chg: dev: Remove memory context form `cfg_obj_t`

Removes the `cfg_obj_t` memory context pointer, as the parser always uses `isc_g_mctx`. This simplifies the parser API/configuration tree API (no need to pass the memory context); and the `cfg_obj_t` size goes down from 80 bytes to 72 bytes.

While not directly related to the changes, also remove the `cfg_parser_t` `references` field as it is not used anymore (since the `cfg_obj_t` types doesn't reference it anymore).

Merge branch 'colin/remove-memctx-cfgobj' into 'main'

See merge request isc-projects/bind9!11199