Remove Unnecessary F-strings (#34)

*Issue #, if available:* N/A

*Description of changes:*
This codemod converts any f-strings without interpolated variables into
regular strings.
In these cases the use of f-string is not necessary; a simple string
literal is sufficient.

While in some (extreme) cases we might expect a very modest performance
improvement, in general this is a fix that improves the overall
cleanliness and
quality of your code.

```diff
- var = f"hello"
+ var = "hello"
  ...
```

<details>
  <summary>More reading</summary>

*
[https://pylint.readthedocs.io/en/latest/user_guide/messages/warning/f-string-without-interpolation.html](https://pylint.readthedocs.io/en/latest/user_guide/messages/warning/f-string-without-interpolation.html)
*
[https://github.com/Instagram/LibCST/blob/main/libcst/codemod/commands/unnecessary_format_string.py](https://github.com/Instagram/LibCST/blob/main/libcst/codemod/commands/unnecessary_format_string.py)
</details>

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src/chronos/chronos.py

@@ -432,7 +432,7 @@ def predict(
         if prediction_length > self.model.config.prediction_length:
             msg = (
                 f"We recommend keeping prediction length <= {self.model.config.prediction_length}. "
-                f"The quality of longer predictions may degrade since the model is not optimized for it. "
+                "The quality of longer predictions may degrade since the model is not optimized for it. "
             )
             if limit_prediction_length:
                 msg += "You can turn off this check by setting `limit_prediction_length=False`."