selected-xxx classes not removed on select updates

Problem/Motivation

In , a coding standards change introduced a bug where selected-... classes from select elements were not correctly removed from the form element.

Steps to reproduce

1. Configure a content type with a term entityreference field with "standard" and "extended" terms, defaulting to "standard"
2. Visit node create form, note that form has class selected-standard
3. Select "extended"
4. Note that class selected-extended has been applied to the form
5. Bug: Note that class selected-standard has not been removed from the form

Proposed resolution

1. Add test coverage to verify expected behaviour (pipeline failing)
2. Revert or fix the coding standards change

Remaining tasks

User interface changes

API changes

Data model changes