Removes two redundant sensors (a raw H:MM:SS string and a duration integer)
in favour of a single SensorDeviceClass.TIMESTAMP that shows the absolute
estimated finish time. The HA frontend auto-renders this as "in X minutes"
and keeps it current without re-polling.
Also removes dead _project / _on_observation / _rem_minutes code from both
operational.py and oven.py — these hooks were declared on the Capability but
never called anywhere in the coordinator or adapter.