In preference fields, do not fallback to displaying the globally preferred value when it's not defined on that preference

Some preferences fields have a TALES for default that displays the preferred value if the value if not defined locally, something like python: context.getPreferredX() or preferences.getPreferredX() .

I believe it is a bad idea, we should just display the value defined on the document like on every other fields.

Git log tells that over their lifetime, these fields have been changed by @tc @jm @kazuhiko @romain @yusei @aurel so please tell me if there was a valid reason for using this pattern that I misunderstood.