Remove 2018 vision page

Now that we're 9 months into FY20, removing the 2018 vision page and associated generator will speed up the build, improve SEO (https://gitlab.com/gitlab-com/marketing/digital-marketing-programs/issues/308), and help avoid confusion. For FY20 we just use an improved version of the direction pages for the same content.

Also found a few other documents that were pointing to the 2018 vision unintentionally.