Remove clever code (AR::Base instantiate)

We had patched AR::Base instantiate method to be able to instantiate miq_workers rows for deleted Worker model classes for the sole purpose of deleting these rows.

Solution:  Delete the rows in a migration and delete the clever code.

AR::Base instantiate's arity changed from 1 in 3.2 to 2 in 4.2 so it's better to just delete this than to make it work with 4.2.

cc @Fryguy @tenderlove 
ooh, yay!  It's been so long since I did migration specs.