The order of the elements is irrelevant and duplicate elements are unwanted. It
is therefore a perfect candidate for a set instead of a list. This prevents
filtering for duplicates manually be replacing some filer codes with (more
performant) builtin methods of QSet.
These were unused (since they were moved to data defined properties
at the symbol layer level) and were not set anywhere in
core QGIS (ie, the only way to set them was by manually calling
this api)
