Remove the tenant-secret association table

Currently all secrets are indirectly associated with tenants via a tenant-secret association table. This association was configured early in the project lifecycle, when it seemed that such an association would aid in retrieving different components of a grouped secret with different RBAC behaviors. However, recent cross-project discussion have indicated that this feature would be better implemented in a different way. Hence this association is adding an additional join on selects with no benefit, and therefore should be removed.