rustdoc: remove no-op CSS .source .content { margin-left: 0 }

This rule originated in 7669f04, to
override the default, massive left margin that content used to accommodate
the sidebar:

https://github.com/rust-lang/rust/blob/7669f04fb0ddc3d71a1fb44dc1c5c00a6564ae99/src/librustdoc/html/static/main.css#L307-L309

This massive left margin doesn't exist any more. It was replaced with a
flexbox-based sidebar layout in 135281e.

Author: notriddle

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

@@ -348,7 +348,6 @@ img {
 .source .content {
 	max-width: none;
 	overflow: visible;
-	margin-left: 0px;
 }
 
 .sub-container {