refactor: Drop unused fclose() from BufferedFile

This was only explicitly used in the tests, where it can be replaced by
wrapping the original raw file pointer into a CAutoFile on creation and
then calling CAutoFile::fclose().

Also, it was used in LoadExternalBlockFile(), where it can also be
replaced by the (implicit call to the) CAutoFile destructor after
wrapping the original raw file pointer in a CAutoFile.

src/streams.h

@@ -637,25 +637,8 @@ public:
         src = fileIn;
     }
 
-    ~BufferedFile()
-    {
-        fclose();
-    }
-
-    // Disallow copies
-    BufferedFile(const BufferedFile&) = delete;
-    BufferedFile& operator=(const BufferedFile&) = delete;
-
     int GetVersion() const { return nVersion; }
 
-    void fclose()
-    {
-        if (src) {
-            ::fclose(src);
-            src = nullptr;
-        }
-    }
-
     //! check whether we're at the end of the source file
     bool eof() const {
         return m_read_pos == nSrcPos && feof(src);