refactor(docs-infra): remove unused styles for `.heading` CSS class (#40427)

This commit removes some styles for the `.heading` CSS class which do
not have any effect (either because they are overridden in other rules
or because they do not make any difference).

NOTE:
The `.heading` class is kept in the HTML to make it easier to associate
the `.heading-children` element with its corresponding `.heading`
parent.

PR Close #40427

aio/src/styles/1-layouts/_sidenav.scss

@@ -106,13 +106,6 @@ button.vertical-menu-item {
   width: 100%;
 }
 
-.heading {
-  color: $darkgray;
-  cursor: pointer;
-  position: relative;
-  @include font-size(16);
-}
-
 .heading-children {
   &.expanded {
     visibility: visible;