remove outdated type-ignores

[kmuehlbauer]

• Closes #xxxx
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[kmuehlbauer]

Those typing issues are a real pain.

So the typing worked in an environment with numpy 2.0.2. It failed with numpy 2.1.3. When removing that type-ignore it resurfaces for numpy dev version. Weird.

• numpy: 2.1.3

xarray/core/dataset.py:9144: error: Unused "type: ignore" comment  [unused-ignore]

• numpy: 2.3.0.dev0+git20250128.a1fa8e1

xarray/core/dataset.py: note: In member "polyfit" of class "Dataset":
xarray/core/dataset.py:9144: error: Incompatible types in assignment (expression has type "floating[Any]", variable has type "float | None")  [assignment]

[kmuehlbauer]

So the floating Any issue is probably this: python/mypy#18566.

Good to merge?