_content: stop the download button from opening /dl in a new tab

Download button opened the dl page in a new tab because previously, the download page was in a separate host (golang.org). Since now go.dev and golang.org are unified to one site, it makes sense to avoid a separate tab.

Fixes golang/go#53637

Change-Id: Ie8c9fd6741be9be154bb375082aff42f7b7d1720
Reviewed-on: https://go-review.googlesource.com/c/website/+/416177
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

_content/index.md

@@ -42,7 +42,6 @@ summary: Go is an open source programming language that makes it easy to build s
         <a class="Primary" href="/learn/">Get Started</a>
         <a class="Secondary js-downloadBtn"
           href="/dl"
-          target="_blank" rel="noopener"
           >Download
         </a>
       </div>
@@ -306,7 +305,7 @@ func main() {
       </p>
       <div class="GettingStartedGo-ctas">
         <a class="GettingStartedGo-primaryCta" href="/learn/">Get Started</a>
-        <a href="/dl" target="_blank" rel="noopener">Download Go</a>
+        <a href="/dl">Download Go</a>
       </div>
     </div>
     <div class="GettingStartedGo-resourcesSection">