temporarily remove education site

[dzimmer]

https://modelica.org/community/education/

This site is basically a collection of dead links.
There is also a Material section with no material.
I think it is better we remove this page until we have something better.

[casella]

Ugh, dead links. Could we set up the CI to crawl them and repot regressions to the webmaster?

[dzimmer]

@t-sommer already provided a scan of dead links in #32
I propose to primarily remove the link to this page. By entering the address directly, it could still be accessed.