Remove token_owner data structure

[llunaCreixent]

The token_owner data structure is not needed anywhere. Furthermore is currently storing the same data value twice (check https://github.com/CirclesUBI/pathfinder2/blob/feature/server_only/src/io.rs#L60). Also, is not necessary in the edges db computation because the value contained in token_owner is already present in the Safe data structure as well as the Balance (check https://github.com/CirclesUBI/pathfinder2/blob/feature/server_only/src/safe_db/db.rs#L8).