btrfs: remove the extent item sanity checks in relocate_block_group

These checks are all taken care of for us by the tree checker code:

- the flags don't change or are updated consistently
- the v0 extent item format is invalid and caught in many other places
  too

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>