refactor(docs-infra): remove unnecessary font-family styles (#40881)

This commit removes some unnecessary styles setting `font-family` to
`$main-font`. These styles are redundant, because the targeted elements
already inherit this style from `<body>`.

PR Close #40881

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

@@ -63,6 +63,7 @@ aio-nav-menu {
       color: $darkgray;
       cursor: pointer;
       display: flex;
+      font-family: $main-font;
       align-items: center;
       justify-content: space-between;
       overflow-wrap: break-word;
@@ -131,7 +132,6 @@ aio-nav-menu {
     }
 
     .level-1 {
-      font-family: $main-font;
       @include font-size(16);
       @include line-height(28);
       font-weight: 400;
@@ -141,7 +141,6 @@ aio-nav-menu {
     }
 
     .level-2 {
-      font-family: $main-font;
       @include font-size(14);
       @include line-height(24);
       font-weight: 400;
@@ -150,7 +149,6 @@ aio-nav-menu {
     }
 
     .level-3 {
-      font-family: $main-font;
       @include font-size(14);
       @include line-height(24);
       margin: 0;
@@ -158,7 +156,6 @@ aio-nav-menu {
     }
 
     .level-4 {
-      font-family: $main-font;
       @include font-size(14);
       @include line-height(24);
       margin: 0;

aio/src/styles/2-modules/_code.scss

@@ -108,7 +108,6 @@ aio-code {
 
       ol.linenums {
         margin: 0;
-        font-family: $main-font;
         color: lighten($mediumgray, 25%);
 
         li {

aio/src/styles/2-modules/_filetree.scss

@@ -7,7 +7,6 @@
 
   .file {
     display: block;
-    font-family: $main-font;
     @include letter-spacing(0.3);
     @include line-height(32);
     color: $darkgray;