dump/files: remove a leftover from dead_pid_conflict

In commit 23b76949 ("dump: optimize dead_pid_conflict by searching
in rbtree") we'd replaced for_each_pstree_item and thread walk search
with pstree_pid_by_virt (rb-tree search). The latter already found us
the thread with conflicting pid if it exists, the only thing left is to
skip if the thread is also a main thread of the thread group (as it was
before patch). But some leftover left which checks something wrong: we
index ->threads array with "i", but "i" is not a number of the thread
it is a number of current dead pid.

Not sure if it helps with the initial bug and duplicates, but it might:
https://jira.sw.ru/browse/PSBM-55217



Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>