Refactor: fencer: Drop stonith_device_t:priority
We've had a TODO to "hook up priority" for over 15 years: c6bac1b3. At
this point, it's safe to skip this and come back to it if needed.
Fencing levels are always an option in the meantime.
Signed-off-by: Reid Wahl <nrwahl@protonmail.com>