kernel: finally remove layer7 filter support

it has been non-functional for years and caused numerous memleaks and
crashes for people that tried to enable it.
it has no maintained upstream source, and it does not look like it's
going to be fixed any time soon

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45423 3c298f89-4303-0410-b956-a3cf2f4a3e73