Drop iconv caching code

Drop iconv caching code

This was introduced for Solaris performance theoretically;
we have never been able to use it on Linux/glibc because
the UTF-16 BOM state isn't reset.

We have no data about Solaris performance; were some to
still exist, we could reintroduce the code with an explicit
check for Solaris, not a check for glibc.

https://bugzilla.gnome.org/show_bug.cgi?id=704999