kde-info2html: get the shared resource dir instead of the (useless) image path

- the third argument of kde-info2html has not been used for ages. It was
originally to be used for the path of images, but it was soon replaced
by the current code which uses help:/... to find the images. Remove it
and the usage of KIconLoader.
- on the other side the usage of kde4-config to get the path for the shared
css is wrong, and kf5-config is in kdelibs4support (deprecated and higher
tier). So find the shared directory in the kio code and recycle the third
argument of the script for this purpose.