[mac] remove unnecessary value store (#6974)
[mac] remove unnecessary value store (#6974)

To remove compiler warning:

  Value stored to 'caps' during its initialization is never read
srickardti/openthread,srickardti/openthread,abtink/openthread,openthread/openthread,jwhui/openthread,srickardti/openthread,abtink/openthread,srickardti/openthread,abtink/openthread,openthread/openthread,jwhui/openthread,jwhui/openthread,jwhui/openthread,abtink/openthread,openthread/openthread,openthread/openthread