Btrfs: always clear a block group node when removing it from the tree

Always clear a block group's rbnode after removing it from the rbtree to
ensure that any tasks that might be holding a reference on the block group
don't end up accessing stale rbnode left and right child pointers through
next_block_group().

This is a leftover from the change titled:
"Btrfs: fix invalid block group rbtree access after bg is removed"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>