rustdoc: remove no-op CSS nav.sub { font-size: 1rem }

This rule originated as a `font-size: 16px`, when body had `font-size: 13px`
set in 4fd061c.

It remained even when body's font size was bumped up to 16px,
4d5f4ff, making the rule a no-op, and was
carried forward when it was converted to 1rem in
cc18120.

Author: notriddle

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

@@ -703,7 +703,6 @@ pre, .rustdoc.source .example-wrap {
 
 nav.sub {
 	position: relative;
-	font-size: 1rem;
 	flex-grow: 1;
 	margin-bottom: 25px;
 }