btrfs: remove unused variable bio_offset from end_bbio_data_read()

The variable @bio_offset was introduced in commit 7ffd27e3 ("btrfs:
pass bio_offset to check_data_csum() directly"), when we are still using
the same endio function for both data and metadata.

Later we had several changes to data and metadata endio functions:

- Data verification is handled by btrfs bio layer

- Split data and metadata endio paths

Now for data path we no longer do any verification in
end_bbio_data_read(), as the verification is handled by btrfs bio layer
already.

Thus there is no need for such bio_offset variable.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>