net_sched: remove qdisc_tree_flush_backlog()

This function is no longer used after the four prior fixes.

Given all prior uses were wrong, it seems better to remove it.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250611111515.1983366-6-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>