block: remove a dead check in show_partition

disk_max_parts never returns 0 given that ->minors for devices not using
the extended dev_t must be non-zero, and disk_max_parts always returns
DISK_MAX_PARTS for the latter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211122130625.1136848-5-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>