Use std::set as duplicate filter in Pcbnew selection moving routine

The filter used before this change did not filter out some duplicates, so it was replaced with an explicit filtering of duplicates with the std::set class.

Fixes #7255 (closed)

I feel this is not the best solution. That bug did not always occur -- its occurence seemed to be random, possibly dependant on the cursor position. So, the root cause may have been somewhere else. But the previous filter didn't appear to be functioning well anyway, and a similar solution to duplication was added in the Page Layout Editor, so I think this may be the right way to solve this problem here as well.