please remove .pnpmfile.cjs for examples

[zhaojc]

A bit more context: .pnpmfile.cjs is no longer generated by Vaadin Flow in 24.2+ versions, so V24.2+ works fine itself. Vaadin 24.1 generates this file, but apparently uses older versions of Node.js/npm/pnpm, with that it doesn't have this buggy behaviour.

The action proposed for Flow is to add this file into vaadin:clean-frontend goal (for Gradle also), and backport this change into all versions that generate this file. So that whenever you migrate to Vaadin 24.2+ you can run mvn clean; mvn vaadin:clean-frontend and avoid this issue.