[XPlane][FG] Q_UNREACHABLE removal follow up

* removed CSimulatorXPlane::connectionFromString as this only works for session DBus
* renamed to "checkConnectionViaSessionBus"
* discussed around here: https://discordapp.com/channels/539048679160676382/539925070550794240/668105458300485652

src/blackmisc/dbusserver.h

@@ -120,6 +120,7 @@ namespace BlackMisc
         static bool isQtDBusAddress(const QString &address);
 
         //! True if address is session or system bus address
+        //! \remark actually swift does NOT support system DBus anymore
         static bool isSessionOrSystemAddress(const QString &address);
 
         //! False if address is session or system bus address