btrfs: Remove unused variable

btrfs: Remove unused variable

Src was initially part of 31ff1cd25d37 ("Btrfs: Copy into the log tree in
big batches"), however 16e7549f045d ("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 <[email protected]>
Reviewed-by: Timofey Titovets <[email protected]>
Reviewed-by: Josef Bacik <[email protected]>
Signed-off-by: David Sterba <[email protected]>