content: bury SwatchBooker

When scrolling over the GIMP home page, examples of programs that can help with the graphic workflow are shown. Among them is SwatchBooker, and without wanting to be disrespectful to the project, it doesn't deserve to be there today.

Although it made sense in the past, its presence currently confuses the user, as it is practically impossible to install the program linked (see details in 6529f51a), which equates it to a dead link, a unwanted web practice.

So, even that very few people get to this part of the page, it's reasonable to remove SwatchBooker, considering his current lack of support (compared to Scribus and Inkscape, which are maintained).