refactor(stepper): remove extra string from some types using StepState

Author: arodr967

src/lib/stepper/step-header.ts

@@ -39,7 +39,7 @@ export class MatStepHeader implements OnDestroy {
   private _intlSubscription: Subscription;
 
   /** State of the given step. */
-  @Input() state: StepState | string;
+  @Input() state: StepState;
 
   /** Label of the given step. */
   @Input() label: MatStepLabel | string;

src/lib/stepper/stepper-icon.ts

@@ -27,7 +27,7 @@ export interface MatStepperIconContext {
 })
 export class MatStepperIcon {
   /** Name of the icon to be overridden. */
-  @Input('matStepperIcon') name: StepState | string;
+  @Input('matStepperIcon') name: StepState;
 
   constructor(public templateRef: TemplateRef<MatStepperIconContext>) {}
 }