VolumeVerifier: Split TMD error from ticket error for WADs

When I first made VolumeVerifier, I figured that the distinction
between an unsigned ticket and an unsigned TMD was a technical
detail that users would have no reason to care about. However,
while this might be true for discs, it isn't equally true for
WADs, due to the widespread practice of fakesigning tickets to
set the console ID to 0. This practice does not require
fakesigning the TMD (though apparently people do it anyway,
at least sometimes...), and the presence of a correctly signed
TMD is a useful indicator that the contents have not been
tampered with, even if the ticket isn't correctly signed.

Source/Core/DiscIO/VolumeVerifier.h

@@ -148,7 +148,6 @@ private:
   std::vector<Partition> CheckPartitions();
   bool CheckPartition(const Partition& partition);  // Returns false if partition should be ignored
   std::string GetPartitionName(std::optional<u32> type) const;
-  void CheckCorrectlySigned(const Partition& partition, std::string error_text);
   bool IsDebugSigned() const;
   bool ShouldHaveChannelPartition() const;
   bool ShouldHaveInstallPartition() const;