Remove ability to configure paperclip in backend

There are numerous drawbacks and "gotchas" in the current image_settings
interface than I argue are worth keeping.

Any changes you make to the settings need a server restart to actually
work. Also, any changes to styles require a rake task to be ran on the
backend. These changes must be completed by programmers and we do not
gain anything supporting them in settings versus setting them in
initializers.

Settings can still be set by using Paperclip::Attachment.default_options,
or can continue to be set on an individual attachment with:

  Spree::Image.attachment_definitions[:attachment][:url] = '/my_url/:id'