rustdoc: remove background-color on .in-band

This was overwriting the style for :target on impl headings. Instead of
showing the whole summary as yellow (on light theme), it would show part
of the summary as white and part as yellow.

Author: jsha

src/librustdoc/html/static/css/themes/ayu.css

@@ -26,10 +26,6 @@ h4 {
 	border: none;
 }
 
-.in-band {
-	background-color: #0f1419;
-}
-
 .invisible {
 	background: rgba(0, 0, 0, 0);
 }

src/librustdoc/html/static/css/themes/dark.css

@@ -13,10 +13,6 @@ h2, h3, h4 {
 	border-bottom-color: #d2d2d2;
 }
 
-.in-band {
-	background-color: #353535;
-}
-
 .invisible {
 	background: rgba(0, 0, 0, 0);
 }

src/librustdoc/html/static/css/themes/light.css

@@ -15,10 +15,6 @@ h2, h3, h4 {
 	border-bottom-color: #DDDDDD;
 }
 
-.in-band {
-	background-color: white;
-}
-
 .invisible {
 	background: rgba(0, 0, 0, 0);
 }