Remove now-unused AthRegSeq
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As discussed on the atlas-sw-core mailing list: https://groups.cern.ch/group/atlas-sw-core/Lists/Archive/Flat.aspx?RootFolder=%2Fgroup%2Fatlas%2Dsw%2Dcore%2FLists%2FArchive%2FAthRegSeq%20used%20anywhere&FolderCTID=0x012002002A6E23175D832F4DAD957CEB680A4D57 AthReqSeq is not used for anything. This MR will remove it. From grepping the repo I see a lot of unit test references: let's see what the CI returns.