flatpak: Don't specify luajit commit

Turns out, even if luajit developers ask people to use branches instead
of tarballs, they sometimes force-push to their stable branches, deleting
commits out of the blue.

Remove the specified commit from the Flatpak manifest.

CI/flatpak/com.obsproject.Studio.json

@@ -146,7 +146,6 @@
           "type": "git",
           "url": "https://luajit.org/git/luajit-2.0.git",
           "branch": "v2.1",
-          "commit": "787736990ac3b7d5ceaba2697c7d0f58f77bb782",
           "disable-shallow-clone": true
         },
         {