Drop form_factor that is appended to JS, CSS and template filenames

fixes #12127

For CSS there is a better approach of detecting the correct form factor. In the current implementation we check for `iphone`, `ipad`, `nokia` and `N9`... it's already a huge amount of devices ... but really?

I checked all core apps. Unit tests run fine locally. Also I found no file (template, CSS or JS) which actually use this functionality.

cc @jancborchardt @DeepDiver1975 @LukasReschke 
PHPDoc still says something about formfactor.