Revert "static/frontend/unit: unit header becomes sticky as the page is scrolled"

This reverts commit 43e917ddc9570bfa168f2c3caccf35c2565218ab.

Reason for revert: golang/go#45841

Change-Id: Ie7939536001be96d11dbe9be565243f84aeae41d
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/341430
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
diff --git a/static/frontend/unit/unit.css b/static/frontend/unit/unit.css
index 309f45d..0fc7e18 100644
--- a/static/frontend/unit/unit.css
+++ b/static/frontend/unit/unit.css
@@ -33,13 +33,13 @@
   grid-area: header;
   min-height: var(--js-unit-header-height);
   padding: 0 var(--gutter);
+  position: sticky;
   top: var(--js-unit-header-top, 0);
   transition: box-shadow 0.25s linear;
   z-index: 10;
 }
 .go-Main-header[data-fixed] {
   border-bottom: none;
-  position: sticky;
 }
 .go-Main-header[data-raised] {
   border-bottom: var(--border);