Remove the roadmap page

We are not keeping the roadmap page up to date, it is currently about a
year behind where it should be with the latest information covering Q3 of
2020. If we are not going to keep it up-to-date, lets get rid of it rather
than present clearly out-of-date information on the website.

The links from all release notes that point to the page are also removed
by this commit, but the text is retained to minimise change to these
historic documents. The text and link has been removed completely from
release schedules.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>