Simplify hovering color styles for headings

The default behavior for headings is to inherit the color, even in the
`:hover` style, and IMHO we can keep it this way, particularly in the
cards, which is the main place where we use headings inside links.

app/assets/stylesheets/layout.scss

@@ -73,15 +73,6 @@ a {
   &:active {
     color: $link-hover;
     text-decoration: underline;
-
-    h1,
-    h2,
-    h3,
-    h4,
-    h5,
-    h6 {
-      color: $link-hover;
-    }
   }
 
   &:focus {
@@ -2770,13 +2761,6 @@ table {
     }
   }
 
-  a:hover {
-
-    h3 {
-      color: #fff;
-    }
-  }
-
   .background-header {
     clip-path: ellipse(60% 80% at 50% 0%);
     height: $line-height * 2;