_content/doc: copypaste.js included more than once

The copypaste js script is included twice, once in the site.tmpl and a second time in install.html.

Change-Id: I5ba7cefa0e4a9f209adebd0cda8c3f855dc40f3f
GitHub-Last-Rev: 6490c1773cf0b19e953be562696fa4482cf5a5b3
GitHub-Pull-Request: golang/website#235
Reviewed-on: https://go-review.googlesource.com/c/website/+/516415
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Nooras Saba‎ <saba@golang.org>
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/_content/doc/install.html b/_content/doc/install.html
index 4930583..6ea21ec 100644
--- a/_content/doc/install.html
+++ b/_content/doc/install.html
@@ -219,4 +219,3 @@
 
 <script async src="/doc/download.js"></script>
 <script async src="/doc/hats.js"></script>
-<script async src="/js/copypaste.js"></script>