cmd/coordinator: remove obsolete build tag

See golang/go#67123

Change-Id: I05d602c1b5decb709473d6f0648837ae4270e9fe
Reviewed-on: https://go-review.googlesource.com/c/build/+/582958
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

cmd/coordinator/buildstatus.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.16 && (linux || darwin)
+//go:build linux || darwin
 
 package main