UBI: factorize destroy_av() and ubi_remove_av() code

Those functions are pretty much doing the same thing, except
ubi_remove_av() is putting the aeb elements attached to the volume into
the ai->erase list and the destroy_av() is freeing them.

Rework destroy_av() to handle both cases.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>