Drupal 10 core stops using *.es6.js files — *.js files can now contain ES6 directly

Thanks to Drupal 10 dropping support for Internet Explorer 11 (#3155358: [policy, no patch] Drop IE11 support from Drupal 10.0.x), all supported browsers now support ECMAScript 6.

This allowed Drupal core to simplify the DX:

1. the *.js file can contain ES6 directly
2. hence the build step (yarn build:js) to turn a *.es6.js file into an ES5 *.js file is no longer necessary.
3. … and all *.es6.js files were removed!