Revert "Revert "Remove everything related to nonexistent $wgExcludeFromThumbnailPurge""

This reverts commit fceeca644000a6cf3484227dc935a02eab51ff06

Setting was actually not included in any release, my mistake
Merge "Revert "Revert "Remove everything related to nonexistent $wgExcludeFromThumbnailPurge"""