Fix test_use_no_fields() not to assert for non-emitted warning

Fix `tests/test_validators.py::test_use_no_fields` not to check
for a warning that can't be emitted because calling the decorator
fails with a `TypeError`.  Starting with pytest 8.0.0 (due to
pytest-dev/pytest#9036 fix), `pytest.warns()` assertions are enforced
even if an exception is raised and consumed by `pytest.raises()`.

Bug pydantic#8674

tests/test_validators.py

@@ -556,11 +556,9 @@ def test_use_no_fields():
         class Model(BaseModel):
             a: str
 
-            with pytest.warns(PydanticDeprecatedSince20, match=V1_VALIDATOR_DEPRECATION_MATCH):
-
-                @validator()
-                def checker(cls, v):
-                    return v
+            @validator()
+            def checker(cls, v):
+                return v
 
 
 def test_use_no_fields_field_validator():