kernel: drop the conntrack rtcache patch

It's in backports-5.4, but it wasn't ever merged. Upstream followed another
approach, with flow offloading, which has much better performance. Drop this
obsolete patch and refresh the kernel patches.

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
Acked-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: Felix Fietkau <nbd@nbd.name>
(cherry picked from commit 17576b1b)