holdings: remove average_cost where it does not make sense
holdings: remove average_cost where it does not make sense

ref #645

yagebu/fava,aumayr/beancount-web,yagebu/fava,aumayr/beancount-web,aumayr/beancount-web,beancount/fava,beancount/fava,yagebu/fava,beancount/fava,beancount/fava,aumayr/beancount-web,yagebu/fava,beancount/fava,yagebu/fava