cmd/link: remove unnecessary use of sync.Once alongside sync.Mutex

cmd/link: remove unnecessary use of sync.Once alongside sync.Mutex

There does not seem to be any point to this sync.Once.
I noticed because I was surveying uses of sync.Once to
understand usage patterns. This seems to be a dreg left over
from some earlier instance of the code.

Change-Id: I99dd258d865a41d0e8f6cfa55887855e477fb9c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/445755
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>