Obsolete the isValid() method in QSerialPortInfo

It has no common use case, so we should get rid of this API in the future. We can deprecate it for now. It has been added for circumvent the scenario when the available ports methods cannot return all the valid ports, but if that is the case, we need to fix that feature rather than adding workaround APIs. Even more, this method cannot help in those cases since it cannot do more than availablePorts().

https://codereview.qt-project.org/#change,72644