Fix user validation error in Import/Export

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23833

Fixes Validation failed: User already exists in source error that appears on import retry attempts.

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

changelogs/unreleased/fix-import-user-validation-error.yml

+---
+title: Remove old project members when retrying an export
+merge_request: 
+author: