btrfs: Remove unused -EIO assignment in end_bio_extent_readpage

In case we hit the error case for a metadata buffer in
end_bio_extent_readpage then 'ret' won't really be checked before it's
written again to. This means the -EIO in this case will never be
checked, just remove it.

Fixes-coverity-id: 1442513
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>