btrfs: remove unused members from struct btrfs_encoded_read_private

The inode and file_offset members in struct btrfs_encoded_read_private
are unused, so remove them.

Last used in commit 7959bd44 ("btrfs: remove the start argument to
check_data_csum and export") and commit 7609afac ("btrfs: handle
checksum validation and repair at the storage layer").

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>