Remove stored state from the redis library

[arturo-seijas]

Issue

#74

Solution

Removes stored state, breaking compatibility

[reneradoi]

The update to the lib version causes the charmcraft pack command to fail with:

Library 'lib/charms/redis_k8s/v0/redis.py' metadata field LIBAPI is different from the version in the path.                                             
Failed to build charm for bases index '0'.

both locally and in CI.

[Mehdi-Bendriss]

I understand that we are breaking backward compatibility by keeping the major version of the lib as is, but we are purposefully doing so as we consider the use of the storedstate object in the relation to be a bug.