chore: remove kwargs from selector function

[MarcoGorelli]

follow-up to #2057 - there's a lot of kwargs we're passing around that we don't need

i'd generally like to refactor that whole mechanism (we'll need it for when we want to support rolling_mean(n).over(group, order_by=id)), but for a start we can remove them when they're not needed

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Related issue #<issue number>
• Closes #<issue number>

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes

If you have comments or can explain your changes, please do so below

[dangotbanned]

I was planning to follow-up with replacing these lambdas:

diff --git a/narwhals/utils.py b/narwhals/utils.py
index cb33a603..05b9cc60 100644
--- a/narwhals/utils.py
+++ b/narwhals/utils.py
@@ -10,6 +10,7 @@ from secrets import token_hex
 from typing import TYPE_CHECKING
 from typing import Any
 from typing import Iterable
+from typing import Iterator
 from typing import Sequence
 from typing import TypeVar
 from typing import Union
@@ -59,6 +60,7 @@ if TYPE_CHECKING:
     from narwhals.typing import DataFrameLike
     from narwhals.typing import DTypes
     from narwhals.typing import IntoSeriesT
+    from narwhals.typing import NativeFrame
     from narwhals.typing import SizeUnit
     from narwhals.typing import SupportsNativeNamespace
     from narwhals.typing import TimeUnit
@@ -1303,6 +1305,14 @@ def dtype_matches_time_unit_and_time_zone(
     )
 
 
+def get_columns(df: NativeFrame) -> Sequence[str]:
+    return df.columns
+
+
+def iter_columns(df: NativeFrame) -> Iterator[str]:
+    yield from df.columns
+
+
 def _hasattr_static(obj: Any, attr: str) -> bool:
     sentinel = object()
     return getattr_static(obj, attr, sentinel) is not sentinel

iter_columns wouldn't be for this part, but would be usuable in lots of places

[MarcoGorelli]

cool, thanks!

[dangotbanned]

Great stuff @MarcoGorelli