cmake: net: qemu: Don't pass "-serial none" to qemu

I don't know why we have been passing "-serial none" to qemu but it
does not seem to be doing anything useful. So I propose we remove it.

According to the qemu documenation it should be disabling all serial
ports;

"-serial dev
Redirect the virtual serial port to host character device dev. The
default device is vc in graphical mode and stdio in non graphical
mode.

This option can be used several times to simulate up to 4 serial
ports.

Use -serial none to disable all serial ports."

But when we use "-serial none", we always combine it with "-serial
pipe", or "-serial unix", to redirect the serial port, so clearly it
is not disabling all serial ports as it attempting to do.

"-serial none" was first introduced to Zephyr in this commit:
0b54f984

As far I can tell, pinging an echo_server works fine without it, so I
suspect that our second -serial flag is overriding it and that it has
no effect.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>