block: remove the bdgrab in blk_drop_partitions

There is no need to hold a bdev reference when removing the partition.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210701081638.246552-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>