PlaylistError: remove unused code "ERRNO"

src/PlaylistError.hxx

@@ -24,7 +24,6 @@ class Domain;
 
 enum class PlaylistResult {
 	SUCCESS,
-	ERRNO,
 	DENIED,
 	NO_SUCH_SONG,
 	NO_SUCH_LIST,

src/command/CommandError.cxx

@@ -36,10 +36,6 @@ print_playlist_result(Response &r, PlaylistResult result)
 	case PlaylistResult::SUCCESS:
 		return CommandResult::OK;
 
-	case PlaylistResult::ERRNO:
-		r.Error(ACK_ERROR_SYSTEM, strerror(errno));
-		return CommandResult::ERROR;
-
 	case PlaylistResult::DENIED:
 		r.Error(ACK_ERROR_PERMISSION, "Access denied");
 		return CommandResult::ERROR;