tst_qfile: Remove unused methods

Remove at() and chmod() methods of MyEngine class. Those methods
are not used anywhere and look like remainder of old API to me.

Change-Id: I754a4281124cb8c9d74e79a9a2b99fb1b1f41e52
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

tests/auto/corelib/io/qfile/tst_qfile.cpp

@@ -2204,9 +2204,7 @@ public:
     MyEngine(int n) { number = n; }
 
     qint64 size() const override { return 123 + number; }
-    qint64 at() const { return -1; }
     QStringList entryList(QDir::Filters, const QStringList &) const override { return QStringList(); }
-    bool chmod(uint) { return false; }
     QString fileName(FileName) const override { return name; }
 
 private: