Remove "Edit this page on GitHub" from outdated doc pages

[bishabosha]

from this comment: #13327 (comment)

leaving the "Edit this page on GitHub" link on the docs, e.g. at https://dotty.epfl.ch/docs/reference/other-new-features/export.html will lead to more PR's being generated here

[BarkingBad]

I don't know if this will be valid after merging #13350 because we will not generate static docs anymore, just preprocess the subset of them for docs.scala-lang website

[julienrf]

We need this before we can merge #13350