static/frontend: removed duplicate aria label for search field.

https://screenshot.googleplex.com/5eMDbj7v64786Lf.png

Change-Id: I74009e80b54511f2ed7822b215140491a6e813d2
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/517395
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: Prudhvi Krishna Chintala