kirkwood: remove support for kernel 4.19

kirkwood has moved to DSA on 5.4, which will make kernel 4.14 and 4.19
support broken on the affected devices.
Support on kernel 5.4 seems to have been running fine for almost 2 months
on multiple devices, while support for 4.19 has never left testing state.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Acked-by: Pawel Dembicki <paweldembicki@gmail.com>