btrfs: remove trivial helper update_nr_written

The helper used to do more with the wbc state but now it's just one
subtraction, no need to have a special helper.

It became trivial in a9132667 ("Btrfs: make mapping->writeback_index
point to the last written page").

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