Deprecate country.default schema and remove the value as it is not used any more

Problem/Motivation

Core no longer uses the system.date:country.default value. We should remove it and deprecate the schema. See #3439439: Remove country setting from the installer and #2276183: Date intl support is broken, remove it

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet