rustdoc: remove no-op CSS h1-6 { border-bottom-color }

For this rule to have an actual effect, the border-bottom width needs
specified, elsewhere, without also specifying the color. This doesn't
happen. Ever since 88b137d, every spot
where headers get a border assigned to them also assigns the color.

Author: notriddle

src/librustdoc/html/static/css/rustdoc.css

@@ -141,10 +141,6 @@ h1, h2, h3, h4 {
 h1.fqn {
 	margin: 0;
 	padding: 0;
-	border-bottom-color: var(--headings-border-bottom-color);
-}
-h2, h3, h4 {
-	border-bottom-color: var(--headings-border-bottom-color);
 }
 .main-heading {
 	display: flex;
@@ -662,9 +658,6 @@ h2.location a {
 
 .docblock h5 { font-size: 1rem; }
 .docblock h6 { font-size: 0.875rem; }
-.docblock h1, .docblock h2, .docblock h3, .docblock h4, .docblock h5, .docblock h6 {
-	border-bottom-color: var(--headings-border-bottom-color);
-}
 
 .docblock {
 	margin-left: 24px;