Revert "make-tarball.nix: Restore the 'commit' field in packages.json"

This reverts commit 5e8545e7.

It breaks eval:

attribute 'rev' missing, at /var/lib/ofborg/checkout/repo/38dca4e3aa6bca43ea96d2fcc04e8229/mr-est/eval-0-gleber.ewr1.nix.ci/pkgs/top-level/make-tarball.nix:106:39