remove remains of Mimetype custom class

A few XWiki classes are annoted to have the custom class org.curriki.xwiki.plugin.mimetype.MimeType.
However, this class is not found by the classloader which the class-editor complains about at each class-editing view, for example at:
http://current.dev.curriki.org/xwiki/bin/edit/Sandbox/B?editor=class

We should drop the documents that contain this custom class reference:

MimeType/animation
MimeType/audio
MimeType/document
MimeType/etc compression files
MimeType/picture
MimeType/video