Change the Fini function to only remove otherwise unreferenced strings

Change the Fini function to only remove otherwise unreferenced strings
from the interned table.  There are references in hard-to-find static
variables all over the interpreter, and it's not worth trying to get
rid of all those; but "uninterning" isn't fair either and may cause
subtle failures later -- so we have to keep them in the interned
table.

Also get rid of no-longer-needed insert of None in interned dict.