ARC: west runner: mdb: remove dead code

Remove code which looking for `cld` process pid - this functionality
isn't use anymore by twister. And as manual MDB termination is also
fixed now this functionality isn't required for anything now.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>