[migration] don't crash when deleting a non-existing rtype
When asked to delete a relation type, instead of checking if it was
computed or not, just delete both the CWRType and CWComputedRType.
for table, column in [
('transactions', 'tx_time'),
('tx_entity_actions', 'tx_uuid'),
('tx_relation_actions', 'tx_uuid')]:
repo.system_source.create_index(session, table, column)
commit()