doc: fix nits in doc/api_assets/style.css

1. Merge rule sets for identical selectors.

2. Delete impossible selector block: we have only stability indexes
   0, 1, and 2, so there can't be `.api_stability_3` class.
   Refs: nodejs.org/api/documentation.html#documentation_stability_index

PR-URL: #20601
Refs: https://nodejs.org/api/documentation.html#documentation_stability_index
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

doc/api_assets/style.css

@@ -29,6 +29,9 @@ h6 { font-size: 1rem }
 
 h1, h2, h3, h4, h5, h6 {
   margin: 1.5rem 0 1rem;
+  text-rendering: optimizeLegibility;
+  font-weight: 700;
+  position: relative;
 }
 
 pre, tt, code, .pre, span.type, a.type {
@@ -183,10 +186,6 @@ ol.version-picker li:last-child a {
   background-color: #4EBA0F;
 }
 
-.api_stability_3 {
-  background-color: #0084B6;
-}
-
 .api_metadata {
   font-size: .85rem;
   margin-bottom: 1rem;
@@ -226,6 +225,8 @@ table {
 
 th, td {
   border: 1px solid #aaa;
+  padding: .75rem 1rem .75rem 1rem;
+  vertical-align: top;
 }
 
 th {
@@ -259,12 +260,6 @@ dd + dt.pre {
   margin-top: 1.6rem;
 }
 
-h1, h2, h3, h4, h5, h6 {
-  text-rendering: optimizeLegibility;
-  font-weight: 700;
-  position: relative;
-}
-
 #apicontent {
   padding-top: 1rem;
 }
@@ -373,9 +368,6 @@ hr {
 #toc .stability_0::after {
   background-color: #d50027;
   color: #fff;
-}
-
-#toc .stability_0::after {
   content: "deprecated";
   margin-left: .25rem;
   padding: 1px 3px;
@@ -493,11 +485,6 @@ span > .mark:hover, span > .mark:focus, span > .mark:active {
   background: none;
 }
 
-th, td {
-  padding: .75rem 1rem .75rem 1rem;
-  vertical-align: top;
-}
-
 th > *:last-child, td > *:last-child {
   margin-bottom: 0;
 }