iconv: Revert steps array reference counting changes

The changes introduce a memory leak for gconv steps arrays whose
first element is an internal conversion, which has a fixed
reference count which is not decremented.  As a result, after the
change in commit 50ce3eae, the steps
array is never freed, resulting in an unbounded memory leak.

This reverts commit 50ce3eae
("gconv: Check reference count in __gconv_release_cache
[BZ #24677]") and commit 7e740ab2
("libio: Fix gconv-related memory leak [BZ #24583]").  It
reintroduces bug 24583.  (Bug 24677 was just a regression caused by
the second commit.)