xds: revert workarounds in tests when go-control-plane issue is fixed

[easwars]

https://github.com/envoyproxy/go-control-plane/issues/431 is a bug in the go-control-plane implementation that results in the server not sending resources in certain cases. We have some workaround in some tests for this. Once the bug is closed, we should revert these workarounds.

So far:

• https://github.com/grpc/grpc-go/blob/2a18bfcb16f3b7961ff28c902753c0b75e7d3ed8/xds/internal/xdsclient/tests/ads_stream_ack_nack_test.go#L477
• https://github.com/grpc/grpc-go/pull/7804