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>

feature/taildrop/delete.go

@@ -12,7 +12,6 @@ import (
 	"time"
 
 	"tailscale.com/ipn"
-	"tailscale.com/syncs"
 	"tailscale.com/tstime"
 	"tailscale.com/types/logger"
 )
@@ -33,7 +32,7 @@ type fileDeleter struct {
 	byName map[string]*list.Element
 
 	emptySignal chan struct{} // signal that the queue is empty
-	group       syncs.WaitGroup
+	group       sync.WaitGroup
 	shutdownCtx context.Context
 	shutdown    context.CancelFunc
 	fs          FileOps // must be used for all filesystem operations