FileSys: remove Open from DirectoryBackend

Open should not be an interface exposed by Directory because it is the Archive thats implement the methed to open the directory. The service API of 3DS also implies this - Open is not a function of directory service, but is of FS main service

src/core/file_sys/ivfc_archive.h

@@ -75,9 +75,6 @@ private:
 
 class IVFCDirectory : public DirectoryBackend {
 public:
-    bool Open() override {
-        return false;
-    }
     u32 Read(const u32 count, Entry* entries) override {
         return 0;
     }