refactor(material/stepper): remove aria-pressed from vertical stepper

Remove unnecessary aria-pressed from vertical stepper.

Author: essjay05

Diff for: src/material/stepper/stepper.html

@@ -73,7 +73,6 @@
             [tabIndex]="_getFocusIndex() === $index ? 0 : -1"
             [id]="_getStepLabelId($index)"
             [attr.aria-expanded]="selectedIndex === $index"
-            [attr.aria-pressed]="selectedIndex == $index"
             [attr.aria-controls]="_getStepContentId($index)"
             [attr.aria-current]="selectedIndex == $index"
             [attr.aria-label]="step.ariaLabel || null"