Remove or fix+test TaxonomyTermReferenceItem's "options_list_callback" setting

Searching HEAD for "options_list_callback" finds 3 matches:
- In the field.taxonomy_term_reference.settings portion of taxonomy.schema.yml.
- In TaxonomyTermReferenceItem::defaultSettings(), where it defaults to NULL.
- In TaxonomyTermReferenceItem::getSettableOptions(), where if it exists, it's invoked.

There appears to be no UI that lets you set it. There appears to be no test that tests it.

Meanwhile, in #2238085: [regression] options_allowed_values() signature doesn't allow for Views filter configuration, we're changing the signature of a similar callback for list fields (which include a UI and tests) to support integration with Views filters and arguments. So, we should make a similar change here if it's a setting we want core to continue to provide. Or, should we just remove the setting entirely?

Tagging "beta target" in case we decide to remove it, which is a config schema change.