ENH: Remove unused namedtuple definition in test

Remove unused `namedtuple` definition in test.

Fixes:
```
test/test_gpr.py:31: error:
 First argument to namedtuple() should be "GradientTablePatch", not "gtab"  [name-match]
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12437972140/job/34728973936#step:8:129

Author: jhlegarreta

test/test_gpr.py

@@ -20,17 +20,13 @@
 #
 #     https://www.nipreps.org/community/licensing/
 #
-from collections import namedtuple
 
 import numpy as np
 import pytest
 from dipy.io import read_bvals_bvecs
 
 from nifreeze.model import gpr
 
-GradientTablePatch = namedtuple("gtab", ["bvals", "bvecs"])
-
-
 THETAS = np.linspace(0, np.pi / 2, num=50)
 EXPECTED_EXPONENTIAL = [
     1.0,