doc/go1.13: remove announcements of arm64 support on NetBSD and OpenBSD

Those configurations currently lack builders, and it is not clear to
me what state their tests are in.

The Go porting policy¹ requires builders for all active ports, so
let's not claim support until that requirement is met.

¹https://golang.org/wiki/PortingPolicy#requirements-for-a-new-port

Updates golang#30824
Updates golang#31656
Updates golang#34035
Updates golang#34036

Change-Id: I6496de9d92fb8546048abf139cf10546a47e314b
Reviewed-on: https://go-review.googlesource.com/c/go/+/192997
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/go1.13.html

@@ -151,18 +151,6 @@ <h3 id="illumos">Illumos</h3>
   build tag.
 </p>
 
-<h3 id="netbsd">NetBSD</h3>
-
-<p><!--CL 155739 -->
-  Go now supports NetBSD on arm64.
-</p>
-
-<h3 id="openbsd">OpenBSD</h3>
-
-<p><!--CL 174125 -->
-  Go now supports OpenBSD on arm64.
-</p>
-
 <h3 id="windows">Windows</h3>
 
 <p><!-- CL 178977 -->