Remove bogus duplicate check

The second condition can never be fulfilled on the first iteration
(QDirIterator does not return a path twice), but only after the
QDirIterator has been reset in the outer loop.

As validThumbnails is incremented when hadFirstThumbnail gets assigned,
the outer loop will never be run after the initial assignement of
hadFirstThumbnail, so the condition can never be true.