Bug 1328985 - Remove project_specific_id compatibility shim

[wlach]

No description provided.

[edmorley]

Running ./manage.py sqlmigrate model 0020 I got:

BEGIN;
--
-- Alter unique_together for job (0 constraint(s))
--
CREATE INDEX `job_repository_id_20bb4d74` ON `job` (`repository_id`);
ALTER TABLE `job` DROP INDEX `job_repository_id_project_specific_id_7883a5e8_uniq`;
COMMIT;

...this is unfortunately buggy since the CREATE INDEX isn't needed, as the existing 5 other index_together entries that start with repository_id cover that field.

As such this removal will need a RunSQL that just does the ALTER TABLE `job` DROP INDEX `job_repository_id_project_specific_id_7883a5e8_uniq`;.