chore(a11y): fix up cdk-visually-hidden class (#6394)

This commit removes the redundant `text-transform` property from the
`cdk-visually-hidden` css class to conform with the *standard* concept
to visually hide content for accessibility.

src/lib/core/a11y/_a11y.scss

@@ -7,7 +7,6 @@
     overflow: hidden;
     padding: 0;
     position: absolute;
-    text-transform: none;
     width: 1px;
   }
 }