_content/css: hide scroll bars under each line number on phone screens

jquery-linedtextarea sets the line numbers to be width 3%
because then it can set the main box to be 97%.
Ideally we'd do something like 3ex and 100% - 3ex
but I've failed to get that to work with the jquery we are using.

On very narrow screens like phones, 3% is too narrow for the
line numbers, and the browser shows a tiny little horizontal
scroll bar under each line number. Make those go away.

Fixes golang/go#49829.

Change-Id: Iccb51a885076d4cbdf91429485960f5ab7790f8b
Reviewed-on: https://go-review.googlesource.com/c/website/+/367976
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Jamal Carvalho <[email protected]>

_content/css/styles.css

@@ -3889,6 +3889,7 @@ h2.Playground-about {
 .linedtextarea .lines div {
   padding-right: 5px;
   color: lightgray;
+  overflow: hidden;
 }
 .linedtextarea .lineerror {
   color: black !important;