Fix leakage of memory context header in find_all_inheritors().

Fix leakage of memory context header in find_all_inheritors().

Commit 827d6f977 contained the same misunderstanding of hash_create's API
as commit 090010f2e.  As in 5d00b764c, remove the unnecessary layer of
memory context.  (This bug is less significant than the other one, since
the extra context would be under a relatively short-lived context, but
it's still a bug.)