chore: remove type embedding TODO from se_r serialize (#3845)

Closes #3554. I just find `se_r` never supports type embedding. It's a
mistake in #3338.

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## Summary by CodeRabbit

- **Refactor**
- Improved the serialization process by removing type embedding
handling, leading to cleaner and more maintainable code.

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Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

deepmd/tf/descriptor/se_r.py

@@ -765,9 +765,6 @@ def serialize(self, suffix: str = "") -> dict:
             raise NotImplementedError("spin is unsupported")
         assert self.davg is not None
         assert self.dstd is not None
-        # TODO: tf: handle type embedding in DescrptSeR.serialize
-        # not sure how to handle type embedding - type embedding is not a model parameter,
-        # but instead a part of the input data. Maybe the interface should be refactored...
         return {
             "@class": "Descriptor",
             "type": "se_r",