kirkwood: drop 4.3 support

target/linux/kirkwood/config-4.3

target/linux/kirkwood/patches-4.3/100-find_active_root.patch

target/linux/kirkwood/patches-4.3/110-ib62x0.patch

target/linux/kirkwood/patches-4.3/120-iconnect.patch

target/linux/kirkwood/patches-4.3/130-dockstar.patch

target/linux/kirkwood/patches-4.3/140-ea4500.patch

target/linux/kirkwood/patches-4.3/150-ea3500.patch

target/linux/kirkwood/patches-4.3/160-goflexhome.patch

target/linux/kirkwood/patches-4.3/170-nsa310s.patch

target/linux/kirkwood/patches-4.3/200-disable-tso.patch