[cmake] remove obsolete efr32 cmake file (#6983)
[cmake] remove obsolete efr32 cmake file (#6983)


srickardti/openthread,srickardti/openthread,openthread/openthread,abtink/openthread,jwhui/openthread,abtink/openthread,abtink/openthread,jwhui/openthread,jwhui/openthread,openthread/openthread,abtink/openthread,srickardti/openthread,openthread/openthread,openthread/openthread,srickardti/openthread,jwhui/openthread