Drop instance_integrations table

instance_integrations table is empty and hasn't been used, since we decided instead of splitting integrations table into integrations and instance_integrations to just set sharding key on the integrations table instead. For this reason, it can be removed.

1. ignore instance_integration_id column in issue/jira/zentao_tracker_data tables
2. drop instance_integration_id column from said tables
3. drop instance_integrations table