`remove_all_fill(tag)` modifies `tag` in place

[gadenbuie]

In #862, as_fill_item() and as_fillable_container() were updated to make a copy of the input tag and to return the modified copy rather than the modified original. However, remove_all_fill(tag) still modifies tag in place.

py-shiny/shiny/ui/fill/_fill.py

Lines 127 to 131 in 6611277

	Returns
	-------
	:
	The original :class:`~htmltools.Tag` object with filling layout attributes
	removed.

We don't use remove_all_fill() internally, but IMHO it should be consistent with the other functions in the module.