copies: delete a redundant cleanup step in _chain() The check is redundant since d5edb5d3a337 (copies: filter out copies when target is not in destination manifest, 2019-02-14). To test that hypothesis, I made this change in the commit that commit, but all tests still passed. I think the case was necessary before then, we just didn't have tests for it. Differential Revision: https://phab.mercurial-scm.org/D6275