find workaround for the missing iconstats files

[derhuerst]

Follow-up of #4607.

Due to the Webpack + plugins upgrade in #4607, I had to remove the iconstats-*.json files because favicon-webpack-plugin@5 doesn't support it anymore. These stats are being used in app/config.js:

digitransit-ui/app/config.js

Line 19 in 0f679ab

stats = require(`../_static/assets/iconstats-${config.CONFIG}`);