cmake: emu: qemu: pkill monitor_15_4

The IEEE 801.15.4 monitor can be started by a parameter to cmake. This
change removes a TODO to kill the monitor after qemu ends.

The change also re-assigns Ctrl-C to Ctrl-D to ensure that QEMU is not
inadvertently sending SIGINT to the surrounding ninja session (which
would leave the monitor sub-process alive).

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>