rustdoc: remove redundant CSS #source-sidebar, #sidebar-toggle

These two elements are always nested below `<nav class="sidebar">`, and will
inherit the font from their parent.

These selectors were added in 93520d2, and
became redundant in 07e3f99 when the source
sidebar elements became nested below `nav.sidebar`.

Author: notriddle

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

@@ -206,7 +206,6 @@ a.source,
 .item-left > a,
 .out-of-band,
 span.since,
-#source-sidebar, #sidebar-toggle,
 details.rustdoc-toggle > summary::before,
 .content ul.crate a.crate,
 a.srclink,