remove prettier config file
remove prettier config file

caused by https://vivliostyle.slack.com/archives/CNN5GPF9V/p1570589074067100

vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle.js