[PP-1] Remove remaining key/value implementation from the entity field map and supporting code

Problem/Motivation

In #3045509: EntityFieldManager key/value field map gets out of sync, doesn't recognise bundle fields we stopped relying on key/value in the entity field map, but left some of the integration in place to support backporting to a patch release.

In this issue, we should be able to remove/deprecate key/value usage in the EntityFieldManager and FieldDefinitionListener classes.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet