workflows: process arxiv math workflow update

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
This may be a bit dangerous considering that there is no guarantee that the snapshot ID will be directly proceeding.

Instead you can make a query, like:

```python
initial_obj = BibWorkflowObject.query.filter(
    BibWorkflowObject.id_parent == obj.id
).one()
initial_obj.delete()
```
Similar to how we did it in tests: https://github.com/inspirehep/invenio/blob/labs/invenio/modules/workflows/testsuite/test_workflows.py#L273

Personally I would like to see a better, more clear, way of handling our snapshots in the future. Now it is sort of like snapshots == ObjectVersion.INITIAL (which would imply it to be a record on it's own.