rustdoc: remove unused CSS #main-content > table td

This rule was added in 4e2c59a970
to benefit the module items table. However, the module items table stopped
using table tags when 6020c79dde
switched us over to grid layout.

You can see when this one used to be triggered by visiting
<https://doc.rust-lang.org/1.54.0/alloc/slice/index.html#structs-1> in a
very narrow window, but it doesn't any more, because the module table is
now rendered using `<div>` tags.

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

@@ -2047,11 +2047,6 @@ in storage.js plus the media query with (min-width: 701px)
 		height: 73px;
 	}
 
-	#main-content > table td {
-		word-break: break-word;
-		width: 50%;
-	}
-
 	#crate-search {
 		border-radius: 4px;
 	}