iov_iter: Remove unused variable in kunit_iov_iter.c

iov_iter: Remove unused variable in kunit_iov_iter.c

Remove the no longer used variable 'b' from iov_kunit_copy_to_bvec().  The
variable is initialised and incremented, but nothing now makes use of the
value.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20260625140640.3116900-9-dhowells@redhat.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
cc: Ming Lei <ming.lei@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: Christoph Hellwig <hch@infradead.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>