eliminate desperate attempts to keep short-long filename mapping complete

*in theory*, there is no way we could at any point know more than gdb
knows and tells us about full path names. let's see what practice shows
for the gdbs we support ...

src/plugins/debugger/gdb/gdbengine.h

@@ -350,8 +350,6 @@ private: ////////// View & Data Stuff //////////
     void handleQuerySources(const GdbResponse &response);
 
     QString fullName(const QString &fileName);
-    // get one usable name out of these, try full names first
-    QString fullName(const QStringList &candidates);
 
     // awful hack to keep track of used files
     QMap<QString, QString> m_shortToFullName;