Remove DTC Postgrads as a category

DTC postgrads do an initial year where they are not assigned to any particular group or even department; they cycle around a few. Because of this there have been issues with them being unable to get a safety induction at Chemistry in order to complete a registration, and Alice Wood has suggested they should instead get signed in as short term visitors for the occasions on which they need to physically enter the department.

Once they finish their initial year and get assigned to a research group in Chemistry they can and should register with us as ordinary postgraduate students.

This change removes all the logic for the DTC postgraduate category from the app. We also need to clean up a reference in the database view registration.crsids_allowed_to_generate_forms in the database although the example SQL here has been updated.