btrfs: Remove BUG_ON from __finish_chunk_alloc()

btrfs_alloc_chunk() unconditionally BUGs on any error returned from
__finish_chunk_alloc() so there's no need for two BUG_ON lines. Remove the
one from __finish_chunk_alloc().

Signed-off-by: Mark Fasheh <mfasheh@suse.de>