STY: Fix lint error in test_where.py (#60206)

Author: mroeschke

pandas/tests/frame/indexing/test_where.py

@@ -4,8 +4,6 @@
 import numpy as np
 import pytest
 
-from pandas._config import using_string_dtype
-
 from pandas.core.dtypes.common import is_scalar
 
 import pandas as pd