lib/fmt: remove even more now-broken uses of FMT_STRING

MPD stopped building since fmt 11.1.0; see
<https://github.com/fmtlib/fmt/issues/4304>. The first commit
fixing this was 9db7144, followed by 5de0909 (both on the
unstable branch).

This commit removes what the author believes to be the remaining
uses in the MPD codebase.

src/lib/icu/Converter.cxx

@@ -41,7 +41,7 @@ IcuConverter::Create(const char *charset)
 	UConverter *converter = ucnv_open(charset, &code);
 	if (converter == nullptr)
 		throw ICU::MakeError(code,
-				     FmtBuffer<256>(FMT_STRING("Failed to initialize charset {:?}"),
+				     FmtBuffer<256>("Failed to initialize charset {:?}",
 						    charset));
 
 	return std::unique_ptr<IcuConverter>(new IcuConverter(converter));
@@ -54,7 +54,7 @@ IcuConverter::Create(const char *charset)
 			iconv_close(to);
 		if (from != (iconv_t)-1)
 			iconv_close(from);
-		throw FmtErrno(e, FMT_STRING("Failed to initialize charset {:?}"),
+		throw FmtErrno(e, "Failed to initialize charset {:?}",
 			       charset);
 	}