rustdoc: remove no-op CSS .source .content { max-width: none }

This rule originated in 7669f04fb0, to
override the default, limited line-width that makes sense for prose, but
doesn't make sense for code (which typically uses hard-wrapped lines):

7669f04fb0/src/librustdoc/html/static/main.css (L153)

This line width limiter isn't applied to the `<div class="content">` node
any more. It's been moved to a separate wrapper `<div>` that used to be
called `main-inner` (in 135281ed15) but is
now called `width-limiter` (since
d7528e2157).

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

@@ -346,7 +346,6 @@ img {
 }
 
 .source .content {
-	max-width: none;
 	overflow: visible;
 }