add migration to drop valid_ensemble_runtype constraitn

[dlebauer]

This constraint should be removed:

ALTER TABLE ensembles ALTER COLUMN runtype SET NOT NULL,
                      ADD CONSTRAINT valid_ensemble_runtype CHECK (runtype IN ('ensemble', 'sensitivity analysis', 'MCMC', 'pda.emulator'));

To drop the table:

ALTER TABLE ensembles DROP CONSTRAINT valid_ensemble_runtype;