feat(angular): remove IonBackButtonDelegate

[sean-perkins]

Issue number: Internal

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What is the current behavior?

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

What is the new behavior?

• Removes IonBackButtonDelegate in favor of IonBackButton

Does this introduce a breaking change?

• Yes
• No

• The IonBackButtonDelegate class has been removed in favor of IonBackButton.

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

Other information

[thetaPC]

LGTM