VolumeVerifier: Remove SystemNotAvailable

We had some code in VolumeVerifier to catch the error message that
Redump.org used to show when trying to access the Wii datfile without
logging in. This restriction was removed from Redump.org around the
start of 2022, and the code has been unnecessary ever since.

Source/Core/DiscIO/VolumeVerifier.cpp

@@ -101,10 +101,6 @@ void RedumpVerifier::Start(const Volume& volume)
     case DownloadStatus::Success:
       return ScanDatfile(ReadDatfile(system), system);
 
-    case DownloadStatus::SystemNotAvailable:
-      m_result = {Status::Error, Common::GetStringT("Wii data is not public yet")};
-      return {};
-
     case DownloadStatus::Fail:
     default:
       m_result = {Status::Error, Common::GetStringT("Failed to connect to Redump.info")};
@@ -121,7 +117,7 @@ static std::string GetPathForSystem(const std::string& system)
 RedumpVerifier::DownloadStatus RedumpVerifier::DownloadDatfile(const std::string& system,
                                                                DownloadStatus old_status)
 {
-  if (old_status == DownloadStatus::Success || old_status == DownloadStatus::SystemNotAvailable)
+  if (old_status == DownloadStatus::Success)
     return old_status;
 
   Common::HttpRequest request;
@@ -141,13 +137,8 @@ RedumpVerifier::DownloadStatus RedumpVerifier::DownloadDatfile(const std::string
   if (result->size() > 1 && (*result)[0] == '<' && (*result)[1] == '!')
   {
     // This is an HTML page, not a zip file like we want
-
-    if (File::Exists(output_path))
-      return DownloadStatus::FailButOldCacheAvailable;
-
-    const bool system_not_available_match =
-        Common::ContainsSubrange(*result, "System \"" + system + "\" doesn't exist.");
-    return system_not_available_match ? DownloadStatus::SystemNotAvailable : DownloadStatus::Fail;
+    return File::Exists(output_path) ? DownloadStatus::FailButOldCacheAvailable :
+                                       DownloadStatus::Fail;
   }
 
   File::CreateFullPath(output_path);

Source/Core/DiscIO/VolumeVerifier.h

@@ -68,7 +68,6 @@ private:
     Success,
     Fail,
     FailButOldCacheAvailable,
-    SystemNotAvailable,
   };
 
   struct DownloadState