fix(list): remove border from last item with item-sliding (#28439)

Issue number: resolves #28435

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## What is the current behavior?
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The item in the last item-sliding still has a border in an inset list.

## What is the new behavior?
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this PR. -->

- Item in last item-sliding no longer has a border

I originally only added `ion-item-sliding:last-of-type ion-item` but I
discovered that the original `ion-item:last-child` causes items in
item-sliding where the item is the last element in the item-sliding
container to not have a border, so the original fix was incomplete.

I added comments as to what each line does and why we didn't just do
`ion-item:last-child`.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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Co-authored-by: Brandy Carney <brandyscarney@users.noreply.github.com>

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Co-authored-by: ionitron <hi@ionicframework.com>
Co-authored-by: Brandy Carney <brandyscarney@users.noreply.github.com>