tanos-jlpt: remove flatten from xml stream

This was earlier used to compensate for a double nesting bug. This has
been fixed in the latest version of the xml package.

lib/_data_ingestion/tanos-jlpt/csv_parser.dart

@@ -10,11 +10,12 @@ Future<List<JLPTRankedWord>> parseJLPTRankedWords(
 ) async {
   final List<JLPTRankedWord> result = [];
 
-  final codec = CsvCodec(
+  final codec = Csv(
     fieldDelimiter: ',',
     lineDelimiter: '\n',
     quoteMode: QuoteMode.strings,
     escapeCharacter: '\\',
+    parseHeaders: false,
   );
 
   for (final entry in files.entries) {
@@ -29,7 +30,6 @@ Future<List<JLPTRankedWord>> parseJLPTRankedWords(
         .openRead()
         .transform(utf8.decoder)
         .transform(codec.decoder)
-        .flatten()
         .map((row) {
           if (row.length != 3) {
             throw Exception('Invalid line in $jlptLevel: $row');