Remove Legacy Github Importer from the codebase
<!--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=235791) </details> <!--IssueSummary end--> Once https://gitlab.com/gitlab-org/gitlab/-/issues/232982 https://gitlab.com/gitlab-org/gitlab/-/issues/225211 https://gitlab.com/gitlab-org/gitlab/-/issues/235764 are complete it should allow us to remove Legacy Github Importer & Client from the codebase since it will no longer be used.
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