lib/fmt: drop use of FMT_STRING

When compiling with libfmt-11.1.0 and newer the following compile errors occur:

In file included from ../src/decoder/DecoderPrint.cxx:23:
../src/client/Response.hxx: In instantiation of 'bool Response::Fmt(const S&, Args&& ...) [with S = decoder_plugin_print(Response&, const DecoderPlugin&)::<lambda()>::FMT_COMPILE_STRING; Args = {const char* const&}]':
../src/decoder/DecoderPrint.cxx:38:7:   required from here
   38 |         r.Fmt(FMT_STRING("plugin: {}\n"), plugin.name);
      |         ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/client/Response.hxx:86:28: error: cannot convert 'const decoder_plugin_print(Response&, const DecoderPlugin&)::<lambda()>::FMT_COMPILE_STRING' to 'fmt::v11::string_view' {aka 'fmt::v11::basic_string_view<char>'}
   86 |                 return VFmt(format_str,
      |                        ~~~~^~~~~~~~~~~~
   87 |                             fmt::make_format_args(args...));
      |                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/client/Response.hxx:81:36: note: initializing argument 1 of 'bool Response::VFmt(fmt::v11::string_view, fmt::v11::format_args)'
   81 |         bool VFmt(fmt::string_view format_str, fmt::format_args args) noexcept;
      |                   ~~~~~~~~~~~~~~~~~^~~~~~~~~~
../src/client/Response.hxx: In instantiation of 'bool Response::Fmt(const S&, Args&& ...) [with S = decoder_plugin_print(Response&, const DecoderPlugin&)::<lambda()>::FMT_COMPILE_STRING; Args = {const char* const&}]':

The error is due to the use of FMT_STRING. The libfmt team shared the following:

    The correct way of using FMT_STRING is to wrap a format string when passing to a
    function with compile-time checks (i.e. that takes format_string) as documented
    in https://fmt.dev/11.1/api/#legacy-compile-time-checks.

    Noting that FMT_STRING is a legacy API and has been superseded by consteval-based
    API starting from version 8: https://github.com/fmtlib/fmt/releases/tag/8.0.0. It
    looks like MPD is trying to emulate {fmt}'s old way of implementing compile-time
    checks which was never properly documented because it was basically a hack. So the
    correct fix is to switch to format_string and, possibly, remove usage of FMT_STRING.

    The old way of doing compile-time checks (fmt::make_args_checked) was documented
    in https://fmt.dev/7.1/api.html#argument-lists but it looks like MPD is not using
    that API so the problematic uses of FMT_STRING have no effect and can just be removed.

The FMT_STRING has been removed in this change based on the fmt-7.1 API and now MPD is
successfully compile against the current libfmt-11.1.0 which highlighted the issue that
had been present in the codebase as it is now triggering the error, is legacy and was
not using the API for which FMT_STRING was aligned with.

src/db/Count.cxx

@@ -49,8 +49,8 @@ PrintSearchStats(Response &r, const SearchStats &stats) noexcept
 	unsigned total_duration_s =
 		std::chrono::duration_cast<std::chrono::seconds>(stats.total_duration).count();
 
-	r.Fmt(FMT_STRING("songs: {}\n"
-			 "playtime: {}\n"),
+	r.Fmt("songs: {}\n"
+	      "playtime: {}\n",
 	      stats.n_songs, total_duration_s);
 }
 

src/db/DatabasePrint.cxx

@@ -49,7 +49,7 @@ static void
 PrintDirectoryURI(Response &r, bool base,
 		  const LightDirectory &directory) noexcept
 {
-	r.Fmt(FMT_STRING("directory: {}\n"),
+	r.Fmt("directory: {}\n",
 	      ApplyBaseFlag(directory.GetPath(), base));
 }
 
@@ -79,10 +79,10 @@ print_playlist_in_directory(Response &r, bool base,
 			    const char *name_utf8) noexcept
 {
 	if (base || directory == nullptr)
-		r.Fmt(FMT_STRING("playlist: {}\n"),
+		r.Fmt("playlist: {}\n",
 		      ApplyBaseFlag(name_utf8, base));
 	else
-		r.Fmt(FMT_STRING("playlist: {}/{}\n"),
+		r.Fmt("playlist: {}/{}\n",
 		      directory, name_utf8);
 }
 
@@ -92,9 +92,9 @@ print_playlist_in_directory(Response &r, bool base,
 			    const char *name_utf8) noexcept
 {
 	if (base || directory == nullptr || directory->IsRoot())
-		r.Fmt(FMT_STRING("playlist: {}\n"), name_utf8);
+		r.Fmt("playlist: {}\n", name_utf8);
 	else
-		r.Fmt(FMT_STRING("playlist: {}/{}\n"),
+		r.Fmt("playlist: {}/{}\n",
 		      directory->GetPath(), name_utf8);
 }
 
@@ -198,7 +198,7 @@ PrintUniqueTags(Response &r, ConstBuffer<TagType> tag_types,
 	tag_types.pop_front();
 
 	for (const auto &[key, tag] : map) {
-		r.Fmt(FMT_STRING("{}: {}\n"), name, key);
+		r.Fmt("{}: {}\n", name, key);
 
 		if (!tag_types.empty())
 			PrintUniqueTags(r, tag_types, tag);