Remove Fallback::iconv()

Remove Fallback::iconv()

The iconv fallback is, for the most part, a remnant of PHP 4 support.
Though iconv was not enabled by default in PHP 4, it is in PHP 5. This
is the case even for Windows builds, which use GNU libiconv.

As for the major Linux distributions:

* Debian, CentOS, Ubuntu -> always enabled
* Fedora, Slackware -> .so in the same package, enabled by default
* Arch -> .so in the same package, disabled by default
* openSUSE -> separate package

Change-Id: Ie1112a5742646a0e1f951e188480c23851859320