Remove the `BONE_UNKEYED` flag. It was only written (set/cleared) but
never actually read.
Also remove `framechange_poses_clear_unkeyed()` as its only function was
to clear the `BONE_UNKEYED` flag. It wasn't used anywhere either.
The only code that used the flag was the `extract_pose_from_action()`,
which was removed in 2869ce6cfa (2009).
No functional changes.