btrfs: block-group: refactor how we delete one block group item
When deleting a block group item, it's pretty straight forward, just
delete the item pointed by the key. However it will not be that
straight-forward for incoming skinny block group item.
So refactor the block group item deletion into a new function,
remove_block_group_item(), also to make the already lengthy
btrfs_remove_block_group() a little shorter.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com> Signed-off-by: Qu Wenruo <wqu@suse.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>