mostPopularVoucherSize.js: Change statistics for pie chart

[MystikHub]

We misinterpreted the statistics that should be shown for this chart. In mostPopularVoucherSize.json, we should be counting and comparing the percentages of the different sizes of voucher sizes purchased (5, 3, 10, etc.) and reporting their percentages. Since the vouchers.json file doesn't report the stores or schemes these voucher packages were purchased for, we can only come up with this statistic on all stores.