rustdoc: remove unnecessary CSS kbd { cursor: default }

Added along with theme picker changes in
e78f1392b79779fa184f9a63e7be04ac7074a1c2, but no reason seems to have been
given at the time for why this particular rule was added.

Removing this rule results in `<kbd>` elements getting an I-bar, while the
rule causes them to use the "default" arrow, but since selecting the text in
these elements works fine, the I-bar is not misleading.

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

@@ -1396,7 +1396,6 @@ kbd {
 	vertical-align: middle;
 	border: solid 1px var(--border-color);
 	border-radius: 3px;
-	cursor: default;
 	color: var(--kbd--color);
 	background-color: var(--kbd-background);
 	box-shadow: inset 0 -1px 0 var(--kbd-box-shadow-color);

src/test/rustdoc-gui/help-page.goml

@@ -27,7 +27,6 @@ define-function: (
             "color": |color|,
             "background-color": |background|,
             "box-shadow": |box_shadow| + " 0px -1px 0px 0px inset",
-            "cursor": "default",
         }, ALL)),
     ],
 )