rustdoc: use the regular arrow indicator for dir-entry CSS

This mostly reverts 468acca, while still
fixing the problem it fixed by using an internal list-style-position. It
results in a slight change in the hover indicator, but nothing misleading.

Author: notriddle

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

@@ -1391,31 +1391,10 @@ details.dir-entry {
 	padding-left: 4px;
 }
 
-details.dir-entry > summary::after {
-	content: " ►";
-	position: absolute;
-	left: -15px;
-	top: 0px;
-	font-size: 80%;
-	padding: 2px 0px;
-	/* set width to cover gap between arrow and text */
-	width: 25px;
-}
-
-details[open].dir-entry > summary::after {
-	content: " ▼";
-}
-
-details.dir-entry > summary::-webkit-details-marker,
-details.dir-entry > summary::marker {
-	display: none;
-}
-
 details.dir-entry > summary {
-	margin: 0 0 0 13px;
-	list-style: none;
+	margin: 0 0 0 -4px;
+	padding: 0 0 0 4px;
 	cursor: pointer;
-	position: relative;
 }
 
 details.dir-entry div.folders, details.dir-entry div.files {