Remove the Navigation Panel from the default menu

This panel is a bit confusing for people for the following reasons:

• It shows all pages at the same level, even important pages. This is why we've just added the new Quick Links panel for important links
• People keep asking how to exclude some pages from the Navigation Panel or how to include only some content
• It doesn't currently scale on big wikis, both in term of performance and in term of presentation
• We have an Index page listing all pages of the wiki, including searches, treeview, etc.