old time field being read in postprocessing

Summary

During post processing it reads the contents of the directory e.g. as volFields. There is no need to automatically read the old-time field as well. This is just an efficiency issue.

Steps to reproduce

Use e.g. setFields. Copy a field to make an old-time one: cp zoneID zoneID_0 Switch on debug flag for volScalarField and run setFields with relevant setFieldsDict. It shows loading the old-time field.