fix(button-toggle): remove unused property and type ToggleType

Author: stevenyxu

src/material/button-toggle/button-toggle.ts

@@ -39,9 +39,6 @@ import {
 } from '@angular/material/core';
 
 
-/** Acceptable types for a button toggle. */
-export type ToggleType = 'checkbox' | 'radio';
-
 /** Possible appearance styles for the button toggle. */
 export type MatButtonToggleAppearance = 'legacy' | 'standard';
 
@@ -411,9 +408,6 @@ export class MatButtonToggle extends _MatButtonToggleMixinBase implements OnInit
    */
   @Input('aria-labelledby') ariaLabelledby: string | null = null;
 
-  /** Type of the button toggle. Either 'radio' or 'checkbox'. */
-  _type: ToggleType;
-
   @ViewChild('button') _buttonElement: ElementRef<HTMLButtonElement>;
 
   /** The parent button toggle group (exclusive selection). Optional. */