[devhelp] readme: flatpak: the nightly version is apparently no longer built

[Sébastien Wilmet <swilmet src gnome org>]

commit 286a73c5a8b8979061acf9dfdc2fbf08c5a30020
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sun Apr 26 20:56:03 2020 +0200

    readme: flatpak: the nightly version is apparently no longer built
    
    It now apparently needs to be built from the GitLab CI, not from
    gnome-apps-nightly.

 README.md | 8 --------
 1 file changed, 8 deletions(-)
---
diff --git a/README.md b/README.md
index d6a85e64..59b06bb7 100644
--- a/README.md
+++ b/README.md
@@ -13,16 +13,8 @@ See the file: [docs/professional-services.md](docs/professional-services.md)
 Installation of the Devhelp Flatpak
 -----------------------------------
 
-### Stable version
-
 - [Devhelp on Flathub](https://flathub.org/apps/details/org.gnome.Devhelp)
 
-### Nightly, development version
-
-```
-$ flatpak install https://gitlab.gnome.org/GNOME/gnome-apps-nightly/raw/master/devhelp.flatpakref
-```
-
 How to contribute
 -----------------