Remove check{,obj_cleanup}_defer documentation

Remove check{,obj_cleanup}_defer documentation

Both check_defer() and the global variable obj_cleanup_defer were removed
from the public API two years ago. Now they were removed from the internals
as well, simplifying the cleanup process greatly. We no longer need them
to have a chance to understand the cleanup process. Also remove references
to EVP_cleanup() since this has long been deprecated and now it doesn't
clean up things anymore.