Simplify implementation by removing some unused code

Only applies to cauldron loader which doesn't exist on Modrinth or
CurseForge, and doesn't even make sense for shaders.

Signed-off-by: TheKodeToad <TheKodeToad@proton.me>

launcher/ui/pages/modplatform/ResourceModel.h

@@ -94,8 +94,7 @@ class ResourceModel : public QAbstractListModel {
     void addPack(ModPlatform::IndexedPack::Ptr pack,
                  ModPlatform::IndexedVersion& version,
                  std::shared_ptr<ResourceFolderModel> packs,
-                 bool is_indexed = false,
-                 QString custom_target_folder = {});
+                 bool is_indexed = false);
     void removePack(const QString& rem);
     QList<DownloadTaskPtr> selectedPacks() { return m_selected; }