Remove obsolete image files

Delete leftovers from long ago: book covers, ads, Rails and beta logos.
Remove files left over from Radiant CMS admin area

* images
* stylesheet
Move image files originating from old layout to own directory
Remove or hide unused logo image files

* Remove unused 'header-ruby-logo.gif'
* Rename old logo with motto, so Jekyll will ignore it
* Remove old low-resolution Ruby logos
Remove legacy book list page (de)
Remove legacy book list page (ko)
Merge pull request #899 from stomar/remove-obsolete-files

Remove obsolete files