libctf: fix ref leak of names of newly-inserted non-root-visible types

A bug in ctf_dtd_delete led to refs in the string table to the
names of non-root-visible types not being removed when the DTD
was.  This seems harmless, but actually it would lead to a write
down a pointer into freed memory if such a type was ctf_rollback()ed
over and then the dict was serialized (updating all the refs as the
strtab was serialized in turn).

Bug introduced in commit fe4c2d55
("libctf: create: non-root-visible types should not appear in name tables")
which is included in binutils 2.35.

libctf/
	* ctf-create.c (ctf_dtd_delete): Remove refs for all types
	with names, not just root-visible ones.