nixos/gerrit: stop setting jvmPackage (#83696)

jdk12_headless disappeared in d00559ebb84af84b48a207a5a0a0762ce9d577eb,
and just using the default in the VM test should be fine IMHO.

Changed files
-1
nixos
tests
-1
nixos/tests/gerrit.nix
···
enable = true;
serverId = "aa76c84b-50b0-4711-a0a0-1ee30e45bbd0";
listenAddress = "[::]:80";
-
jvmPackage = pkgs.jdk12_headless;
jvmHeapLimit = "1g";
plugins = [ lfs ];