url_downlaoder: Remove QNetworkDiskCache caching

The Qt network caching doesn't work for our needs since it doesn't necessarily
get teh latest file from the server nad using "AlwaysNetwork" defeats the purpose
of caching.

In the future, custom caching will be introduced that fits Multipass's needs.

Fixes #825

src/network/url_downloader.cpp

@@ -40,20 +40,11 @@ namespace
 {
 constexpr auto category = "url downloader";
 
-auto make_network_manager(const mp::Path& cache_dir_path)
+// TODO: We will be using our own caching in the future due to the issue
+//       seen in https://github.com/CanonicalLtd/multipass/issues/825
+auto make_network_manager(const mp::Path& /* cache_dir_path */)
 {
-    auto manager = std::make_unique<QNetworkAccessManager>();
-
-    if (!cache_dir_path.isEmpty())
-    {
-        auto network_cache = new QNetworkDiskCache;
-        network_cache->setCacheDirectory(cache_dir_path);
-
-        // Manager now owns network_cache and so it will delete it in its dtor
-        manager->setCache(network_cache);
-    }
-
-    return manager;
+    return std::make_unique<QNetworkAccessManager>();
 }
 
 template <typename ProgressAction, typename DownloadAction, typename ErrorAction, typename Time>
@@ -101,6 +92,22 @@ mp::URLDownloader::URLDownloader(std::chrono::milliseconds timeout) : URLDownloa
 mp::URLDownloader::URLDownloader(const mp::Path& cache_dir, std::chrono::milliseconds timeout)
     : cache_dir_path{QDir(cache_dir).filePath("network-cache")}, timeout{timeout}
 {
+
+    if (!cache_dir_path.isEmpty())
+    {
+        // Clean up old QNetworkDiskCache dirs
+        QDir data_dir{QDir(cache_dir_path).filePath("data8")};
+        if (data_dir.exists())
+        {
+            data_dir.removeRecursively();
+        }
+
+        QDir prepared_dir{QDir(cache_dir_path).filePath("prepared")};
+        if (prepared_dir.exists())
+        {
+            prepared_dir.removeRecursively();
+        }
+    }
 }
 
 void mp::URLDownloader::download_to(const QUrl& url, const QString& file_name, int64_t size, const int download_type,