btrfs: don't save block group root into super block

The extent tree v2 needs a new root for storing all block group items,
the whole feature hasn't been finished yet so we can afford to do some
changes.

My initial proposal years ago just added a new tree rootid, and load it
from tree root, just like what we did for quota/free space tree/uuid/extent
roots.

But the extent tree v2 patches introduced a completely new way to store
block group tree root into super block which is arguably wasteful.

Currently there are only 3 trees stored in super blocks, and they all
have their valid reasons:

- Chunk root
  Needed for bootstrap.

- Tree root
  Really the entry point for all trees.

- Log root
  This is special as log root has to be updated out of existing
  transaction mechanism.

There is not even any reason to put block group root into super blocks,
the block group tree is updated at the same time as the old extent tree,
no need for extra bootstrap/out-of-transaction update.

So just move block group root from super block into tree root.

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