fix(stepper): remove tabindex from content

A while ago in #14892 we added a `tabindex` to the content of the current step, however since then we consulted with the a11y team and it looks like adding the `tabindex` there isn't the best approach.

Fixes #16692.

Author: crisbeto

src/material/stepper/stepper-horizontal.html

@@ -27,7 +27,6 @@
 
 <div class="mat-horizontal-content-container">
   <div *ngFor="let step of steps; let i = index"
-       [attr.tabindex]="selectedIndex === i ? 0 : null"
        class="mat-horizontal-stepper-content" role="tabpanel"
        [@stepTransition]="_getAnimationDirection(i)"
        (@stepTransition.done)="_animationDone.next($event)"

src/material/stepper/stepper-vertical.html

@@ -23,7 +23,6 @@
 
   <div class="mat-vertical-content-container" [class.mat-stepper-vertical-line]="!isLast">
     <div class="mat-vertical-stepper-content" role="tabpanel"
-         [attr.tabindex]="selectedIndex === i ? 0 : null"
          [@stepTransition]="_getAnimationDirection(i)"
          (@stepTransition.done)="_animationDone.next($event)"
          [id]="_getStepContentId(i)"