fix(docs-infra): remove redundant `MatIcon` element (#42584)

Previously, we included a dummy `MatIcon` element in `AppComponent` in
order to ensure that the `Material Icons` font would be requested (and
thus cached by the ServiceWorker) on every navigation. However, #41129
introduced the `ThemeToggleComponent`, which will be present on all
pages (since it is located in the top-menu) and relies on the
`Material Icons` font. Therefore, the work-around for loading the
`Material Icons` font is no longer necessary.

This commit removes the now redundant `MatIcon` from `AppComponent`.

PR Close #42584

aio/src/app/app.component.html

@@ -70,13 +70,3 @@
 <footer class="no-print">
   <aio-footer [nodes]="footerNodes" [versionInfo]="versionInfo"></aio-footer>
 </footer>
-
-<!--
-  Work-around to ensure that the `.woff2` file for the 'Material Icons' font will always be fetched.
-  This allows the SW to cache the file (even if there are no icons in the current page) and ensure
-  material icons on other pages are displayed correctly in offline mode.
-  (Note: Instantiate lazily to avoid affecting the initial rendering.)
--->
-<div class="cdk-visually-hidden" *ngIf="!isStarting">
-  <mat-icon>&nbsp;</mat-icon>
-</div>

goldens/size-tracking/aio-payloads.json

@@ -3,7 +3,7 @@
     "master": {
       "uncompressed": {
         "runtime-es2017": 4619,
-        "main-es2017": 456646,
+        "main-es2017": 456422,
         "polyfills-es2017": 55210,
         "styles": 69324,
         "light-theme": 79417,
@@ -15,7 +15,7 @@
     "master": {
       "uncompressed": {
         "runtime-es2017": 4619,
-        "main-es2017": 456785,
+        "main-es2017": 456561,
         "polyfills-es2017": 55348,
         "styles": 69324,
         "light-theme": 79417,