Don't think target is best name here.

Is this needed?

Not entirely sure how the bbones are stored internally, but usually in the linked list you would do something like:

pchan->bbone_prev = target_pchar->bbone_prev

Or maybe even:

target_pchar->bbone_prev->bbone_next = target_pchar->bbone_next;
target_pchar->bbone_next->bbone_prev = target_pchar->bbone_prevt;

But i am not sure whether bbone chain thingamajig is supposed to "break" on removing pchan or not.
Not even sure if pchan can be part of multiple bbone chains.

It's not a linked list I think, just a cache to quickly lookup the next/prev bone for b-bones. So I don't think this part is an issue.

Ok, then ignore my comment :)

What would be a better name? Does unlinked_pchansounds better?

Sounds good to me.