Clean up iconv-related patches.

The port is still linking to libiconv.so from ports on all FreeBSD releases,
but that will be dealt with separately.

For now:

• Remove patch-cmake_FindIconv.cmake. Since the iconv port is always present (sometimes due to an indirect dependency), the hacks in the patch are not necessary and the ports files are always found (this will change later and libc will be preferred on >= 10.1).
• Make patch-CMakeLists.txt smaller. It is fine to let the port check for sys/iconv.h, and the try_run() statement can be fixed on 9.3 by making sure the test program is built with iconv's paths and libraries.

Test-built on 9.3-i386, 10.1-amd64 and HEAD-amd64.