btrfs: Remove unused variable

Src was initially part of 31ff1cd2 ("Btrfs: Copy into the log tree in
big batches"), however 16e7549f ("Btrfs: incompatible format change
to remove hole extents") changed parameters passed to copy_items which
made the src variable redundant.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Timofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>