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

[easwars]

envoyproxy/go-control-plane#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:

• grpc-go/xds/internal/xdsclient/tests/ads_stream_ack_nack_test.go

  Line 477 in 2a18bfc

  // TODO(https://github.com/envoyproxy/go-control-plane/issues/1002): Once

• xds/resolver: fix flaky test TestResolverRemovedWithRPCs with a workaround #7804

[purnesh42H]

@easwars do we need to keep this open if nothing is expected from us? Can we tag it as blocked?