Remove unused iconv.h includes

These are left over from 3e4d684, which
added includes to *all* platforms despite adding bindings only to *some*
of them. This already broke OpenBSD which doesn't have iconv.h (fixed by
915d8fa), and is just distasteful, so
down with those unused includes.

libc-test/build.rs

@@ -559,7 +559,6 @@ fn test_redox(target: &str) {
         "errno.h",
         "fcntl.h",
         "grp.h",
-        "iconv.h",
         "limits.h",
         "locale.h",
         "netdb.h",
@@ -620,7 +619,6 @@ fn test_solarish(target: &str) {
         "fcntl.h",
         "glob.h",
         "grp.h",
-        "iconv.h",
         "ifaddrs.h",
         "langinfo.h",
         "limits.h",
@@ -1334,7 +1332,6 @@ fn test_android(target: &str) {
                "errno.h",
                "fcntl.h",
                "grp.h",
-               "iconv.h",
                "ifaddrs.h",
                "limits.h",
                "locale.h",
@@ -1922,7 +1919,6 @@ fn test_emscripten(target: &str) {
                "fcntl.h",
                "glob.h",
                "grp.h",
-               "iconv.h",
                "ifaddrs.h",
                "langinfo.h",
                "limits.h",