btrfs: remove io_failure_record::in_validation

The io_failure_record::in_validation was introduced to handle failed bio
which cross several sectors.  In such case, we still need to verify
which sectors are corrupted.

But since we've changed the way how we handle corrupted sectors, by only
submitting repair for each corrupted sector, there is no need for extra
validation any more.

This patch will cleanup all io_failure_record::in_validation related
code.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>