rustdoc: remove no-op CSS #settings-menu { padding: 0 }

This CSS was added in 5e01ba36c9f1037c4cf3e7421413fc6c41f85d05, and served to
override CSS right above it that set a 5px padding for several kinds of
buttons in the same toolbar.

The CSS that it overrode is still there, but now it only applies to
`#settings-menu > a`, so there's nothing to override.

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

@@ -1449,9 +1449,7 @@ pre.rust {
 	border-radius: 2px;
 	cursor: pointer;
 }
-#settings-menu {
-	padding: 0;
-}
+
 #settings-menu > a, #help-button > button {
 	padding: 5px;
 	height: 100%;