syncs: delete WaitGroup and use sync.WaitGroup.Go in Go 1.25

Our own WaitGroup wrapper type was a prototype implementation
for the Go method on the standard sync.WaitGroup type.
Now that there is first-class support for Go,
we should migrate over to using it and delete syncs.WaitGroup.

Updates #cleanup
Updates tailscale/tailscale#16330

Change-Id: Ib52b10f9847341ce29b4ca0da927dc9321691235
Signed-off-by: Joe Tsai <joetsai@digital-static.net>

cmd/tailscale/cli/file.go

@@ -20,6 +20,7 @@ import (
 	"path"
 	"path/filepath"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"time"
 	"unicode/utf8"
@@ -32,7 +33,6 @@ import (
 	"tailscale.com/envknob"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/tsaddr"
-	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
 	tsrate "tailscale.com/tstime/rate"
 	"tailscale.com/util/quarantine"
@@ -176,7 +176,7 @@ func runCp(ctx context.Context, args []string) error {
 			log.Printf("sending %q to %v/%v/%v ...", name, target, ip, stableID)
 		}
 
-		var group syncs.WaitGroup
+		var group sync.WaitGroup
 		ctxProgress, cancelProgress := context.WithCancel(ctx)
 		defer cancelProgress()
 		if isatty.IsTerminal(os.Stderr.Fd()) {