btrfs: delete stale comments after merge conflict resolution

There are two comments in btrfs_cache_block_group that I left when
resolving conflict between commits ced8ecf0 "btrfs: fix space cache
corruption and potential double allocations" and 527c490f "btrfs:
delete btrfs_wait_space_cache_v1_finished".

The former reworked the caching logic to wait until the caching ends in
btrfs_cache_block_group while the latter only open coded the waiting.
Both removed btrfs_wait_space_cache_v1_finished, the correct code is
with the waiting and returning error. Thus the conflict resolution was
OK.

Signed-off-by: David Sterba <dsterba@suse.com>