btrfs: remove hole check in prealloc_file_extent_cluster

btrfs: remove hole check in prealloc_file_extent_cluster

Extents in the extent cluster are guaranteed to be contiguous as such
the hole check inside the loop can never trigger. In fact this check was
never functional since it was added in 8520469721c3 ("btrfs: update
btrfs_space_info's bytes_may_use timely") which came after the commit
introducing clustered/contiguous extents 5e906cb14963 ("Btrfs: relocate
file extents in clusters").

Let's just remove it as it adds noise to the source.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>