README: Drop "Steam as a Flatpak" section

This points to a Github repository that is no longer up, and was never
as polished as the version of Steam maintained by the Flathub community.
Signed-off-by: Simon McVittie <smcv@collabora.com>

README

@@ -61,8 +61,3 @@ On the same machine, or a test machine onto which you have copied
     flatpak run org.debian.packages.hello
     flatpak --user install flatdeb org.debian.packages.openarena
     flatpak run org.debian.packages.openarena
-
-Bonus content: Steam as a Flatpak, with multiarch
--------------------------------------------------
-
-https://github.com/smcv/flatdeb-steam