btrfs: remove unused define WAIT_PAGE_LOCK for extent io

Last use was in the readahead code that got removed by f26c9238
("btrfs: remove reada infrastructure").

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>