net: sched: remove qdisc_watchdog->last_expires

This field mirrors hrtimer softexpires, we can instead
use the existing helpers.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230308182648.1150762-1-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>