fix: decouple weixin build helpers from whatsapp

pkg/channels/whatsapp.go

@@ -287,14 +287,3 @@ func nextBackoff(current, max time.Duration) time.Duration {
 	}
 	return next
 }
-
-func sleepWithContext(ctx context.Context, d time.Duration) bool {
-	timer := time.NewTimer(d)
-	defer timer.Stop()
-	select {
-	case <-ctx.Done():
-		return false
-	case <-timer.C:
-		return true
-	}
-}