Cleanup iconv build system flags

[equalsraf]

iconv support was reenabled in #1370 but the iconv detection and code could use some cleanup. A quick grep through the code shows several iconv related flags

1. USE_ICONV is defined in src/nvim/iconv.h and can be true if we are linked against iconv or if we support dynamic loading of iconv
2. HAVE_ICONV_H is defined by config/CMakeLists.txt if the header is found
3. likewise HAVE_ICONV is defined config/CMakeLists.txt, but it is based on the cmake check in CMakeLists.txt (see Iconv_FOUND)
4. DYNAMIC_ICONV is meant for loading the iconv dll at runtime (even if the header is not available at build time), but it is never set in Neovim - which makes sense for now since this one is only meant for Windows (mbyte.c). [RFC] remove DYNAMIC_ICONV #10708

Do we want to continue supporting DYNAMIC_ICONV? Vim uses dynamic loading and distributes the iconv dll separately - If not, the code can be removed, if we want to keep it we need to check some Vim changes (vim/vim/issues/440).

At least some of this can be simplified in CMakeLists.txt and config/CMakeLists.txt if the FindIconv.cmake is changed to handle the header flag. @justinmk this last bit can be tagged as entry level.

[justinmk]

We should distribute iconv with the Windows build.

[brcolow]

This looks interesting:
https://github.com/mlocati/gettext-iconv-windows