Remove Drive objects from objectstore schema

The DriveRegister and DriveState objects were moved from the objectstore to the DB a long time ago, but the old definitions still exist in the objectstore schema (objectstore/cta.proto). They should be removed.

(Incidentally the actual objects still exist in the production objectstore, last accessed in 2021: rados -p tapecta --id cta-production --namespace cta-production ls |grep "^DriveState".)