Undo database change to delta file.

Accidentally made a chanage to the delta_5 file that upgraded the
databases from 2x to 3x, problem is that I drop the field in question
delta_6, so it has to be there in delta_5 or stuff will break.
Change data type constraits on mixeval_questions.

mixeval_question_type_id should never be null, otherwise, we don't know
what type of question to treat this entry as.