chore: remove module from fga.mod definition

Description

We initially added a module field to the fga.mod file, however after some thinking I'm proposing that we remove this field for the following:

It doesn't actually do anything and can get erased as we never transfer it over to the transformed model (i.e. a modular model is reconstructed from the JSON then it will be lost unlike schema version and contents).
There doesn't seem to be (to me) a reason for it's existence, each module file will contain a module name and in the current iteration there doesn't seem to be any value to having a top-level module name. Whilst one might exist in the future I feel that attempting to design for that now might paint us into a corner as we do not know any future needs right now.

[x] I have clicked on "allow edits by maintainers".
[ ] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
[x] The correct base branch is being used, if not main
[ ] I have added tests to validate that the change in functionality is working as expected