btrfs: Cleanup num_tolerated_disk_barrier_failures

As we use per-chunk degradable check, the global
num_tolerated_disk_barrier_failures is of no use.

We can now remove it.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>