content/static: re-enable target highlighting

Now that the directory tree layout is active we can
re-enable target highlighting.

Fixes golang/go#43773

Change-Id: Ie29c5428e18938b8b99fd3920b3b17ca3f448965
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/287635
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>

content/static/css/unit_details.css

@@ -48,12 +48,9 @@
   }
 }
 
-/*
-TODO(https://golang.org/issue/43773): fix highlight bug.
 .UnitDetails :target:not(h2) {
   background-color: var(--yellow);
 }
-*/
 .UnitDetails-outline {
   display: none;
   height: calc(100vh - 7.8475rem);