次のことを試してください（redis-cliでこれらすべてを検証できます）：
yesterday.empty() # This will wipe out rq:queue:yesterday and all of its contents
del(yesterday) # Deletes the variable itself
r = Redis()
r.srem('rq:queues', 'rq:queue:yesterday') # Removed the entry from rq:queues set. The library unfortunately doesn't seem to clean this up by itself.