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/command/OtherCommands.cxx

@@ -377,7 +377,7 @@ handle_config(Client &client, [[maybe_unused]] Request args, Response &r)
 #endif
 
 	if (const auto spl_path = map_spl_path(); !spl_path.IsNull())
-		r.Fmt(FMT_STRING("playlist_directory: {}\n"), spl_path.ToUTF8());
+		r.Fmt("playlist_directory: {}\n", spl_path.ToUTF8());
 
 #ifdef HAVE_PCRE
 	r.Write("pcre: 1\n");