STY: Remove duplicate NiBabel import

Remove duplicate `NiBabel` import.

Fixes:
```
Shadows name 'nb' from outer scope
```

raised locally by the IDE.

Author: jhlegarreta

src/nifreeze/estimator.py

@@ -173,8 +173,6 @@ def run(self, dataset: DatasetT, **kwargs) -> Self:
 
             bmask_path = None
             if dataset.brainmask is not None:
-                import nibabel as nb
-
                 bmask_path = ptmp_dir / "brainmask.nii.gz"
                 nb.Nifti1Image(
                     dataset.brainmask.astype("uint8"), dataset.affine, None