qemuarm64-secureboot: don't set append in extlinux.conf

There's no need to set append in the extlinux.conf, as the kernel will
pull the settings passed from runqemu via the DeviceTree.

By explicitly setting append here the kernel doesn't look in the DT, and
the extlinux.conf hardcoded a TAP network configuration which meant that
networking fails for runqemu using slirp.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/wic/qemuarm64.cfg

@@ -1,4 +1,3 @@
 default Yocto
 label Yocto
     kernel /Image
-append root=/dev/vda2 rw console=ttyS0 mem=1024M ip=192.168.7.2::192.168.7.1:255.255.255.0 console=ttyAMA0