_content: do not run playground code on page load

The purpose is to avoid overwriting user content restored by opening
recently closed tab from the history.

It turns out that https://golang.org/cl/407055 does not address the
issue reliably. If the run action happens before the user content is
restored, the source code of the "hello world" example will be sent
to the server for formatting. Most of the time (if not always), the
format result will return after the user content is restored. In this
case, the format result will overwrite the restored user content.

We can delay the run action until the user content is restored. But it's
subtle to decide how long to delay. On the other hand, I think it does
not make sense to run the "hello world" example on load, so I prefer
to address the issue by removing the run action on page load.

Fixes golang/go#49895
Fixes golang/go#53065

Change-Id: Ia044103e79a15d0b351e4257af3aa74558c302d9
GitHub-Last-Rev: 40170ad2e8
GitHub-Pull-Request: golang/website#162
Reviewed-on: https://go-review.googlesource.com/c/website/+/408414
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

_content/js/playsite.js

@@ -23,7 +23,6 @@ window.addEventListener('load', () => {
 
     // The pre matched below is added by the code above. Style it appropriately.
     document.querySelector(".js-playgroundOutputEl pre").classList.add("Playground-output");
-    $('.js-playgroundRunEl').click();
 
     $('#code').linedtextarea();
     $('#code').attr('wrap', 'off');