btrfs: remove now unused checksumming helpers

Remove the unused btrfs_verify_data_csum helper, and fold
btrfs_check_data_csum into its only caller.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>