Remove checking of OneToOneField type in determining primary key name.

[sezabart]

Tripping over the same issue that was documented here back in 2013 and without a solution, I decided to get rid of the error-causing clause and live dangerously.

In my experience this works, but there might be edge cases (@vdboor mentions one) that this is supposed to protect against, which it won't now. But at least I can use my OneToOneField's