refactor: remove validation functions and tests for return processes

- Deleted validation functions for electronic devices, nonbook items, and ton/datentraeger.
- Removed associated test files for these validations.
- Updated question definitions to use new constants for item conditions and return reasons.
- Refactored return process service to utilize schema validation instead of custom validators.
- Adjusted HTML templates to reflect changes in eligibility state handling.

apps/isa-app/stories/ui/input-controls/checklist.stories.ts

@@ -12,7 +12,7 @@ import {
   CheckboxAppearance,
 } from '@isa/ui/input-controls';
 import { provideAnimations } from '@angular/platform-browser/animations';
-import { Component, importProvidersFrom } from '@angular/core';
+import { importProvidersFrom } from '@angular/core';
 
 interface ChecklistStoryProps {
   values: string[];