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

This rule was added to override non-zero left margin on `.content`,
which was removed in 135281e and
the margin-left was put on the docblock.

Author: notriddle

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

@@ -1655,10 +1655,6 @@ in storage.js
 		margin-top: 1em;
 	}
 
-	.content {
-		margin-left: 0px;
-	}
-
 	.anchor {
 		display: none !important;
 	}