rustdoc: remove unneeded position: relative setting CSS

This was added to control percentage sizes, in
79956b9

Now, the only percentage size is [`border-radius`], which is
based on the size of the box itself, not its containing block.
This leaves the property unused.

[`border-radius`]: https://developer.mozilla.org/en-US/docs/Web/CSS/border-radius

src/librustdoc/html/static/css/settings.css

@@ -1,6 +1,5 @@
 .setting-line {
 	margin: 1.2em 0.6em;
-	position: relative;
 }
 
 .setting-radio input, .setting-check input {

tests/rustdoc-gui/settings.goml

@@ -301,7 +301,7 @@ wait-for-css: ("#help-button .popover", {"display": "block"})
 // Now we go to the settings page to check that the CSS is loaded as expected.
 go-to: "file://" + |DOC_PATH| + "/settings.html"
 wait-for: "#settings"
-assert-css: (".setting-line", {"position": "relative"})
+assert-css: (".setting-radio", {"cursor": "pointer"})
 
 assert-attribute-false: ("#settings", {"class": "popover"}, CONTAINS)
 compare-elements-position: (".sub form", "#settings", ("x"))
@@ -322,4 +322,4 @@ reload:
 set-window-size: (300, 1000)
 click: "#settings-menu"
 wait-for: "#settings"
-assert-css: (".setting-line", {"position": "relative"})
+assert-css: (".setting-radio", {"cursor": "pointer"})