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

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 #30824
Updates #31656
Updates #34035
Updates #34036

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