How can I restore ##MS_SSISServerCleanupJobLogin##?
I decided to experiment with creating the user with a random password, and everything worked.

CREATE LOGIN [##MS_SSISServerCleanupJobLogin##] WITH PASSWORD=N'blahblahblah', DEFAULT_DATABASE=[master], DEFAULT_LANGUAGE=[us_english], CHECK_EXPIRATION=OFF, CHECK_POLICY=OFF
GO

USE SSISDB
GO
ALTER USER [##MS_SSISServerCleanupJobUser##] with LOGIN = [##MS_SSISServerCleanupJobLogin##]