btrfs: get rid of extent_io_tree::dirty_bytes

This was used as an optimization for count_range_bits(EXTENT_DIRTY),
which was used by the failed record code.  However this was removed in
this series by patch "btrfs: convert the io_failure_tree to a plain
rb_tree" which was the last user of this optimization.  Remove the
->dirty_bytes as nobody cares anymore.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>