QmlDebugClient: make QmlDebugConnection::flush() private

There is no reason for any external class to access that. If the call
to flush() from the qml profiler had any effect then it hid a bug that
should be fixed.

Change-Id: I8f655733bd3bff808653cd56b1904aa87c31f6c0
Reviewed-by: Kai Koehne <kai.koehne@digia.com>