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>

syncs/syncs.go

@@ -402,19 +402,3 @@ func (m *Map[K, V]) Swap(key K, value V) (oldValue V) {
 	mak.Set(&m.m, key, value)
 	return oldValue
 }
-
-// WaitGroup is identical to [sync.WaitGroup],
-// but provides a Go method to start a goroutine.
-type WaitGroup struct{ sync.WaitGroup }
-
-// Go calls the given function in a new goroutine.
-// It automatically increments the counter before execution and
-// automatically decrements the counter after execution.
-// It must not be called concurrently with Wait.
-func (wg *WaitGroup) Go(f func()) {
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
-		f()
-	}()
-}