Bug 16019 - Remove unused blue.css

Bug 16019 - Remove unused blue.css

This patch removes blue.css and one image which was used only by
blue.css, css_header.png.

To test, apply the patch and search Koha files for references to
blue.css and css_header.png. There should be none.

Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com