Deleted asset should not be accessible via REST

[c8y3]

The goal of this PR was to fix the following regression: after an asset was deleted, it was not anymore in the list of assets of the case, but was still accessible with its identifier.
This problem was made visible because we don't rely on both the asset identifier and case identifier to retrieve an asset anymore. But, there was still some unexpected behavior in the existing code.
More specifically here https://github.com/dfir-iris/iris-web/blob/v2.4.14/source/app/datamgmt/case/case_assets_db.py#L128, case_asset.alerts is always different from None (because it is an empty list), and the asset was never really removed from the assets data table. Only it's case_id was set to null. (the code from here https://github.com/dfir-iris/iris-web/blob/v2.4.14/source/app/datamgmt/case/case_assets_db.py#L138, was never reachable)

I decided to always remove the asset completely. I am not sure this can have side-effects somewhere. I tried to explore all possible scenarios and made multiple tests. I made this fix in an isolated PR, so that you can fix in some other way, if necessary. The test scenarios will still be useful.
Also, this was the occasion to fix a missing case for IOC deletion.

The new tests cover these scenarios:

• test_get_asset_should_return_404_after_it_was_deleted
• test_delete_asset_should_increment_asset_state
• test_delele_asset_should_not_fail_when_it_is_linked_to_an_ioc
• test_delete_asset_should_not_fail_when_it_has_associated_comments
• test_delete_asset_should_delete_associated_comments
• test_delete_ioc_should_not_prevent_case_deletion