build(flatpak): remove unused source JSON files from manifest

- Remove `flatpak-sources-convention.json` and `flatpak-sources-manual.json` from the build sources.

Author: rainxchzed

packaging/flatpak/zed.rainxch.githubstore.yml

@@ -137,5 +137,3 @@ modules:
         dest-filename: gradle-bin.zip
       # Pre-downloaded Gradle/Maven dependencies for offline build
       - flatpak-sources.json
-      - flatpak-sources-convention.json
-      - flatpak-sources-manual.json