Remove PackageArchive::getAllExistingRequirements()

Remove PackageArchive::getAllExistingRequirements()

The last remaining user of this method was removed in the previous commit. The
logic of this method was overly complex:

- It handled the possibility that a single package is installed multiple times,
  which is not possible since forever.
- It contained special (and semi-broken) logic for requirement links that are
  already stored in the database. Since a93b160e72731d72e58a13052c1b6b83b089552a
  all requirements in the database were deleted before this method was called
  in PackageInstallationDispatcher. It also failed to account for a requirement
  link existing, but the target package being of an insufficient version.