For consistency the folder icons used for the unclustered / clustered items is supposed to be the Qt system folder icon SP_DirIcon.

There was a fallback implemented in https://github.com/metabrainz/picard/commit/d0322f8285da82e93b888845b160875bfe99a1e7 for Qt 4.1 compatibility. With the change to PyQt scoped enums in https://github.com/metabrainz/picard/commit/c5662606cc08fe270c4f266a89d2c42928a8b52f#diff-56adc91512cd2edb211f7983807dcf9e0bd7d8f89dfbac7e2c3053c0de10f60bR247 this check was not properly updated, but still worked in Qt5.

With Qt6 the check broke, and now always the fallback icon is used.

Since we do not need to care about Qt 4.1 compatibility anymore, the check can be dropped.