chore: delete the inert .prettierrc (#495)

Nextcloud itself uses no prettier — nextcloud/server and nextcloud/text have no
prettier dependency, no format script and no prettier config; they ship
.editorconfig and enforce JS/Vue formatting through @nextcloud/eslint-config.

In this repo the file never ran: no prettier dependency, no format script, no
workflow reference. It only took effect in editors, where its 2-space indent and
double quotes are exactly what @nextcloud/eslint-config then flags.

Author: rubenvdlinde