doc: printk-formats: do not mention casts for u64/s64

Do not mention casts for u64/s64.  Complements commit 2a7930bd
("Documentation/printk-formats.txt: No casts needed for u64/s64").

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>