SEP 11.x stores 3 content revisions, while 12.1 now only stores 1.
That being said I believe these should be removed on uprgade.
You can probably delete them however, I would call Symantec and open a case. There was a utility called Rx4defs that was used in 11.x. I think it was used for corrupt defs but you should call. Not sure if there is one for 12.1 as I've never had to use.
See this thread on obtaining
https://www-secure.symantec.com/connect/forums/rx4defssep-utility