Remove validation support

[jelhan]

Ember File Upload currently supports validation of files per mime type and extension. But this feature has some bugs and unexpected behaviors:

• <FileUpload> may allow a file to be uploaded, which fails validation rules enforced by <FileInput>.
  On <FileUpload> it only sets accept HTML attribute. It does not validate if the given file matches the validation after it's selected. Some browser allow the user to ignore the accept list.
  <FileDropzone> validates all files and blocks upload if not passing accept criteria.
• Due to browser limitations validation of files provided to <FileDropzone> via drag and drop happens too late.
• Provided validation is inflexible if multiple files are given, out of which some are valid and some are invalid.
• It misses advanced validation features like resetting validation state.

Due to this reasons I propose that we drop that feature. Instead we should provide APIs to plugin validation by consumers.

Previous discussions can be found in #127 and #376

As this would be a fundamental change affecting capabilities of this addon I would like to have great consent before moving forward.

[lukemelia]

As the author of #127, I support this change, provided that there is a documented upgrade path.

[gilest]

@jelhan I agree that we should drop this feature