common: Require an installed libiconv.

* common/utf8conv.c: Remove dynload.h.
(load_libiconv): Remove.  Remove all calls to it.

The iconv functions are standard feature on most systems and in any
case libiconv can be used to provide the functions. The old code used
to dlopen iconv.dll on Windows. This goes back to GnuPG-1 which was
designed as a one-binary program without any hard dependencies.
GnuPG2 however demands a lot of libraries anyway and thus there is no
more need for the fragile code to load a possible wrong version of
iconv.dll at runtime.