"The original code is broken to begin with, as it ignores the return values from
the std::unique calls, so excess elements remain at the end of va and vb."
(<http://lists.freedesktop.org/archives/libreoffice/2013-January/043552.html>
"[PATCH] Simplify MigrationImpl::subtract in desktop;" see that mail thread also
for the inspiration to use std::set_difference.)

This fix is not very relevant though, as there are no "ExcludedFiles" lists in
officecfg/registry/data/org/openoffice/Setup.xcu, so the second argument is
always empty, so the return value is always a (sorted) copy of the first
argument, and the "IncludedFiles" lists in Setup.xcu produce no duplicates, so
std::unique does not shrink the first argument anyway.

Change-Id: Ie9fb64ee40fef6e7bdf0f5d0eca5717fec7b0d50