So far I found no way to remove widgets completely.
GC appears to not do anything about them either as long as your level is running.
This goes of course for dynamically created buttons etc. as well, so if you want to make an inventory that is not constant, every time you “remove” an item, it will stay with you forever.
So all you can do is use the already created ones and only create new ones if you need one.
I really hope they add a delete widget node eventually. After all this time I doubt it though.