dashboard: remove known issue for darwin arm builder

The builder successfully passes all.bash and produces binary releases.

Updates golang/go#42756
Fixes golang/go#39782

Change-Id: I554a1e3541d185958103e11dcc4b2514445d8275
Reviewed-on: https://go-review.googlesource.com/c/build/+/278772
Trust: Alexander Rakoczy <alex@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>

dashboard/builders.go

@@ -2302,7 +2302,6 @@ func init() {
 			// Darwin ARM64 added in Go 1.16.
 			return atLeastGo1(goBranch, 16) && defaultPlusExpBuild(repo, branch, goBranch)
 		},
-		KnownIssue: 39782,
 	})
 	addBuilder(BuildConfig{
 		Name:           "darwin-amd64-race",