copies: delete misplaced comment
The comment was added in
78d760aa3607 (duplicatecopies: do not mark
items not in the dirstate as copies, 2013-03-28). It became misplaced
in
3666331164bb (cmdutil: add copy-filtering support to
duplicatecopies, 2014-06-07). Then the relevant code was moved far
away in
754b5117622f (context: add workingfilectx.markcopied,
2017-10-15).
Differential Revision: https://phab.mercurial-scm.org/D6352
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