Utils: remove unused variable in SaveFile class

Backup member was never set to true.

Change-Id: I21dc889813820e2a463c5ba812b18b57766e03c4
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

src/libs/utils/savefile.h

@@ -44,14 +44,11 @@ public:
     void rollback();
     bool commit();
 
-    void setBackup(bool backup) { m_backup = backup; }
-
     static void initializeUmask();
 
 private:
     const QString m_finalFileName;
     bool m_finalized;
-    bool m_backup;
     static QFile::Permissions m_umask;
 };