btrfs: remove unused logic of limiting async delalloc pages

A proposed patch in https://marc.info/?l=linux-btrfs&m=147859791003837


pointed out bad limit threshold in cow_file_range_async, but it turned
out that the whole logic is not necessary and is done by writeback. We
agreed to remove it.

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