Allow for metadata fields to be removed

### Description

(edited by @rvl)

There are currently only CLI options to add metadata fields.

It may help users if there were CLI options to remove metadata fields from the json file.

For example:

```
cardano-metadata-submitter 19309eb9c066253cede617dc635223ace320ae0bbdd5bd1968439cd0 --remove-url
```

### Workaround

Either edit the JSON manually or delete the JSON file and start over.

### Original report

Using `--unit` results in a "unit" object being added to the metadata json. This can be modified but not removed using the tool. It would be nice to add an option to `--unit 0`, or similar, to remove this section from the metadata json.

Start with the cardano-metadata-submitter CLI option handling and the code that updates the metadata JSON file. Review how existing options such as --unit and --remove-url would be represented, then verify that a removal option deletes the selected field or object without affecting other metadata. Done means users no longer need to edit or delete the JSON manually.

Written by the indexing model from the issue text.