feat(angular): remove IonBackButtonDelegate (#29030)

Issue number: Internal

---------

<!-- Please do not submit updates to dependencies unless it fixes an
issue. -->

<!-- Please try to limit your pull request to one type (bugfix, feature,
etc). Submit multiple pull requests if needed. -->

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

We currently export two versions of the `ion-back-button` angular
implementation: `IonBackButton` and `IonBackButtonDelegate`.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Removes `IonBackButtonDelegate` in favor of `IonBackButton`

## Does this introduce a breaking change?

- [x] Yes
- [ ] No

<!--
  If this introduces a breaking change:
1. Describe the impact and migration path for existing applications
below.
  2. Update the BREAKING.md file with the breaking change.
3. Add "BREAKING CHANGE: [...]" to the commit description when merging.
See
https://github.com/ionic-team/ionic-framework/blob/main/.github/CONTRIBUTING.md#footer
for more information.
-->

- The `IonBackButtonDelegate` class has been removed in favor of
`IonBackButton`.

  ```diff
  - import { IonBackButtonDelegate } from '@ionic/angular';
  + import { IonBackButton } from '@ionic/angular';
  ```



## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->