Migrator to remove designated_class slot

[brynnz22]

This migrator removes the designated_class slot from all collections that use it.

This migrator was missed because there was no specific PR that mentioned it, but I believe it was probably part of adding the type slot requirement PR. I labeled this migrator as unknown. Let me know if you want to change it.

[eecavanna]

I'm OK with unknown in the name for now (given that it's the only migrator with that in its name).

I will be renaming all the migrators soon so that their order is indicated by their names. I'm still deciding on a naming convention, but it may be something like this:

migrator_from_10_2_0_to_11_0_0_A.py
migrator_from_11_0_0_A_to_11_0_0_B.py
migrator_from_11_0_0_B_to_11_0_0_C.py
# ...
migrator_from_11_0_0_K_to_11_0_0.py

[eecavanna]

I reviewed this draft PR. Its contents so far, look good to me.

[eecavanna]

Looks good to me!