<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=521996) </details> <!--IssueSummary end--> After removing the `gitea_user_mapping` FF in https://gitlab.com/gitlab-org/gitlab/-/issues/512211, we can remove the old user mapping code from Gitea. This issue is to remove the old user mapping code which is behind the code check [ProjectImportData#user_mapping_enabled?](https://gitlab.com/gitlab-org/gitlab/-/blob/ed395ea54bd6097c1e56b8840ee7b2fccb067610/app/models/project_import_data.rb#L50-52)