Revert "FindIconv: add ICONV_CMAKE_OBJECTS_FLAGS variable"

The only thing that cmake objects requires on compiling is -DLIBICONV_PLUG,
so there is no need in extra COMPILE_FLAGS (in fact linker will fail with it).
Just call FindIconv() before creating cmake objects, and that's all.

This reverts commit 6df86d057941ba3140b70f14435d20434802e0e2.