Remove slice from conditional join test

Since we don't have row order guarantees we won't use this.

python/cudf_polars/tests/test_join.py

@@ -128,14 +128,7 @@ def test_join_literal_key_unsupported(left, right, left_on, right_on):
         [pl.col("b") <= pl.col("a_right") * 7, pl.col("a") < pl.col("d") * 2],
     ],
 )
-def test_join_where(left, right, conditions, zlice):
+def test_join_where(left, right, conditions):
     q = left.join_where(right, *conditions)
 
     assert_gpu_result_equal(q, check_row_order=False)
-
-    if zlice is not None:
-        q_len = q.slice(*zlice).select(pl.len())
-        # Can't compare result, since row order is not guaranteed and
-        # therefore we only check the length
-
-        assert_gpu_result_equal(q_len)