Is it possible to to clear the the owner attribute value in WI type Defect every time the defect is opened for edit?

This is of questionable merit, IMO. 

1. Artifically forces judgment on any potential change, which may not be directly related to the current owner ( e.g. adding a remark )
   
2. could cause excessive notifications to parties no longer "interested" as assigning owner I think also assigns that person as a subscriber, definitely the person saving the work item will be subscribed.  This would leave quite the trail
   

Any time a work item is viewed it is potentially modifiable.  Data present on the work item is subject to validation and operation behavior(s) when the Save action is taken.   RTC has at least 1 attribute ( Filed Against ) whose intent is to at least initially triage the work item.

no, this is not possible with the default build-in customization settings or pre and post condition

how to get this:
write an extension
there are a lot of examples on jazz net or various blogs of smart customers