flatpak: Don't specify luajit commit

luajit developers ask people to use branches instead of tarballs, however,
Flatpak interprets having both 'commit' and 'branch' fields as 'use this
branch, and this commit should be at the top of the branch', which is not
really what we want.

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
         },
         {