Remove ember-in-element-polyfill dependency

[felixbuenemann]

This removes the ember-in-element-polyfill since it causes deprecations on Ember.js 5.12 and is no longer needed:

DEPRECATION: The original property on literal nodes is deprecated, use value instead

I recommend a major version bump, since it breaks older Ember version, that don't support {{in-element}} natively yet.