[Improvement] ShuffleBlockInfo.getSize can be optimized when enabling Netty using off-heap memory

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/incubator-uniffle/issues?q=is%3Aissue) and found no similar issues.

### What would you like to be improved?

As the title says, we use off-heap memory when enabling Netty, so there is no need to add the metadata size of fields.

### How should we improve?

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

Locate ShuffleBlockInfo.getSize and trace how it is used when Netty uses off-heap memory. Confirm the size calculation does not include field metadata on that path, then verify the resulting size behavior with the relevant existing tests or callers; the issue does not name a specific file or test.

Written by the indexing model from the issue text.