context: delete an obsolete comment

Obsolete since 91618801d5c3 (context: raise ProgrammingError on
repo['my-tag'], 2018-07-06).

Differential Revision: https://phab.mercurial-scm.org/D4002

mercurial/context.py

@@ -438,7 +438,6 b' class changectx(basectx):'
                         "unsupported changeid '%s' of type %s" %
                         (changeid, type(changeid)))
 
-            # lookup failed
         except (error.FilteredIndexError, error.FilteredLookupError):
             raise error.FilteredRepoLookupError(_("filtered revision '%s'")
                                                 % pycompat.bytestr(changeid))