fix(back-button): show correct background on focus + hover with ios (#27723)

Issue number: resolves #27722

---------

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

The back button on iOS uses the same background and foreground colors on
hover + focus making it impossible to read

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

- Back button uses a transparent background color on hover + focus to
match the behavior of `ion-button`

| `main` | branch |
| - | - |
| <video
src="https://github.com/ionic-team/ionic-framework/assets/2721089/e8bed785-66d7-40cd-beb3-54636c7fc59a"></video>
| <video
src="https://github.com/ionic-team/ionic-framework/assets/2721089/2c4a9a5c-f1a9-4a83-879f-8b0e9ce18558"></video>
|

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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screenshots of how the component looks before and after the change. -->

core/src/components/back-button/back-button.ios.scss

@@ -38,11 +38,3 @@
     opacity: .6;
   }
 }
-
-
-// Back Button Color: Focused
-// --------------------------------------------------
-
-:host(.ion-color.ion-focused) .button-native::after {
-  background: #{current-color(base)};
-}