Code Quality: Deleted ItemViewModel.ApplySingleFileChangeAsync

[hishitetsu]

This prevents a loophole in #14567.
ApplyFilesAndFoldersChangesAsync is used even for a single file change in most cases, and there is no rational reason to leave ApplySingleFileChangeAsync.

Resolved / Related Issues

• Were these changes approved in an issue or discussion with the project maintainers? In order to prevent extra work, feature requests and changes to the codebase must be approved before the pull request will be reviewed. This prevents extra work for the contributors and maintainers.
  Related to Fix: Fixed issue where the app could crash when adding items from external apps #14567

Validation
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• Did you build the app and test your changes?
• Did you check for accessibility? You can use Accessibility Insights for this.
• Did you remove any strings from the en-us resource file?
  • Did you search the solution to see if the string is still being used?
• Did you implement any design changes to an existing feature?
  • Was this change approved?
• Are there any other steps that were used to validate these changes?

[yaira2]

LGTM