extensions-app: Stop prefixing .desktop file

Adding prefixes like (Nightly) is discouraged nowadays in favor of
the radioactive icon overlays.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3612

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1661>

subprojects/extensions-app/build-aux/flatpak/org.gnome.Extensions.json

@@ -5,7 +5,6 @@
   "sdk": "org.gnome.Sdk",
   "command": "gnome-extensions-app",
   "tags": ["nightly"],
-  "desktop-file-name-prefix": "(Nightly) ",
   "finish-args": [
     "--share=ipc", "--socket=fallback-x11",
     "--socket=wayland",