btrfs: delete btrfs_wait_space_cache_v1_finished

We used to use this in a few spots, but now we only use it directly
inside of block-group.c, so remove the helper and just open code where
we were using it.

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