merge: remove workaround for issue5020 As I explained in the previous commit, I think the filtering added there is a better fix for the issue, so the workaround from 41f6af50c0d8 (merge: fix crash on criss cross merge with dir move and delete (issue5020), 2017-01-31) should no longer be needed. Differential Revision: https://phab.mercurial-scm.org/D6245