[EHR] unable to deselect employer in occupational medicine

[ValeriyDyachenko]

Enables clearing the "Employer - Occupational Medicine" field.

1. Centralized removable fields logic - REMOVABLE_FIELDS constant and isRemovableField() helper control which fields support deletion. Easy to extend for other fields.

2. Explicit clear token - Cleared fields send answer: [] (tracked via dirtyFields) to signal intentional deletion vs. empty field.

3. Hidden field protection - filterHiddenRemovableFields() prevents accidental deletion when field is hidden by enableWhen conditions, not explicitly cleared by user.