DiscIO: Remove m_partition and m_volume from FileSystem

The last commit made m_partition unnecessary, and m_volume
has been unnecessary ever since the PR that added DiscExtractor.

Source/Core/DiscIO/Filesystem.cpp

@@ -3,18 +3,11 @@
 // Refer to the license.txt file included.
 
 #include "DiscIO/Filesystem.h"
-#include <memory>
-#include "DiscIO/Volume.h"
 
 namespace DiscIO
 {
 FileInfo::~FileInfo() = default;
 
-FileSystem::FileSystem(const Volume* volume, const Partition& partition)
-    : m_volume(volume), m_partition(partition)
-{
-}
-
 FileSystem::~FileSystem() = default;
 
 }  // namespace