FileSys: remove Open from FileBackend

Same as directory, file shouldn't expose Open either.

src/core/file_sys/file_backend.h

@@ -18,12 +18,6 @@ public:
     FileBackend() {}
     virtual ~FileBackend() {}
 
-    /**
-     * Open the file
-     * @return Result of the file operation
-     */
-    virtual ResultCode Open() = 0;
-
     /**
      * Read data from the file
      * @param offset Offset in bytes to start reading data from