⚡️ Remove the redundant stream transform (#2037)

No issues were fixed by this.

### New Pull Request Checklist

- [x] I have read the
[Documentation](https://pub.dev/documentation/dio/latest/)
- [x] I have searched for a similar pull request in the
[project](https://github.com/cfug/dio/pulls) and found none
- [x] I have updated this branch with the latest `main` branch to avoid
conflicts (via merge from master or rebase)
- [ ] I have added the required tests to prove the fix/feature I'm
adding
- [ ] I have updated the documentation (if necessary)
- [x] I have run the tests without failures
- [x] I have updated the `CHANGELOG.md` in the corresponding package

### Additional context and info (if any)

No semantic changes during the request.

dio/CHANGELOG.md

@@ -5,7 +5,7 @@ See the [Migration Guide][] for the complete breaking changes list.**
 
 ## Unreleased
 
-*None.*
+- Improve `SyncTransformer`'s stream transform.
 
 ## 5.3.4
 

dio/lib/src/transformers/sync_transformer.dart

@@ -62,37 +62,24 @@ class SyncTransformer extends Transformer {
       return responseBody;
     }
 
-    final showDownloadProgress = options.onReceiveProgress != null;
     final int totalLength;
-    if (showDownloadProgress) {
+    if (options.onReceiveProgress != null) {
       totalLength = int.parse(
         responseBody.headers[Headers.contentLengthHeader]?.first ?? '-1',
       );
     } else {
       totalLength = 0;
     }
 
-    int received = 0;
-    final stream = responseBody.stream.transform<Uint8List>(
-      StreamTransformer.fromHandlers(
-        handleData: (data, sink) {
-          sink.add(data);
-          if (showDownloadProgress) {
-            received += data.length;
-            options.onReceiveProgress?.call(received, totalLength);
-          }
-        },
-      ),
-    );
-
     final streamCompleter = Completer<void>();
     int finalLength = 0;
     // Keep references to the data chunks and concatenate them later.
     final chunks = <Uint8List>[];
-    final subscription = stream.listen(
-      (chunk) {
+    final subscription = responseBody.stream.listen(
+      (Uint8List chunk) {
         finalLength += chunk.length;
         chunks.add(chunk);
+        options.onReceiveProgress?.call(finalLength, totalLength);
       },
       onError: (Object error, StackTrace stackTrace) {
         streamCompleter.completeError(error, stackTrace);