Simplify device registry (#77715)

* Simplify device registry

* Fix test fixture

* Update homeassistant/helpers/device_registry.py

Co-authored-by: epenet <6771947+epenet@users.noreply.github.com>

* Update device_registry.py

* Remove dead code

Co-authored-by: epenet <6771947+epenet@users.noreply.github.com>