Old fart left several handguns to his son,  another old fart.  Old fart jr has since left the state of CA and wants to sell the inherited property.
Are there any laws to "unregister" these handguns previously purchased/owned possibly carried concealed in the state of CA?

My guess would be the only way to get something off of a list is to transfer it to another list, so to speak.   Or more accurately transfer via an ffl.  Follow Ca law if transferring to a Ca resident or Federal law if transferred to an out of state resident. Transfers as the result of an inheritance are pretty common, I'd expect.  Short of creating a legal record of some sort, the last known owner would be the Ca. resident because they were DROS'd at the time of purchase. (Assuming they were recent enough legal transfers.)  The alternative would be to not follow the laws (not allowed as a recommendation here, of course) with the expectation that if the subject were to somehow arise formally, they be found to have been registered to Old Fart, Sr., in California and now in the hands of someone else.

IIRC, transfers, or filing no-longer-in-possession forms, does not remove the association of the firearm with a person's name in the database.

There is the question of when the inheritance took place.  If it was in the 1990s or earlier, very good chance that the firearms are not registered in Jr.'s name, and some or even all may not be registered at all.

Old fart left several handguns to his son, another old fart. Old fart jr has since left the state of CA and wants to sell the inherited property.

Dude has left California and wants to sell his firearms.  He is no longer subject to California State law.

Are there any laws to "unregister" these handguns previously purchased/owned possibly carried concealed in the state of CA?

You and he have no responsibilities beyond complying with your State of North Carolina laws and the Federal laws.