uki.bbclass: drop serial console from kernel command line

uki.bbclass: drop serial console from kernel command line

The kernel will continue using console from firmware which is
much better on HW when we may not know at build time which
console HW and drivers are available, e.g. like on
genericarm64 machine.

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>