net/sched: taprio: replace safety precautions with comments

The WARN_ON_ONCE() checks introduced in commit 13511704

 ("net:
taprio offload: enforce qdisc to netdev queue mapping") take a small
toll on performance, but otherwise, the conditions are never expected to
happen. Replace them with comments, such that the information is still
conveyed to developers.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>