improve server->client tcp performance for high latency links

similar to the other fix, if we do not set the buffer size manually,
we get better performance for high latency connections

restore benchmark from f.gruenbicher:

no delay, without patch: ~50MB/s
no delay, with patch: ~50MB/s
25ms delay, without patch: ~11MB/s
25ms delay, with path: ~50MB/s

my own restore benchmark:

no delay, without patch: ~1.5GiB/s
no delay, with patch: ~1.5GiB/s
25ms delay, without patch: 30MiB/s
25ms delay, with patch: ~950MiB/s

for some more details about those benchmarks see
https://lists.proxmox.com/pipermail/pbs-devel/2020-September/000600.html

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/client/http_client.rs

@@ -292,7 +292,6 @@ impl HttpClient {
 
         let mut httpc = hyper::client::HttpConnector::new();
         httpc.set_nodelay(true); // important for h2 download performance!
-        httpc.set_recv_buffer_size(Some(1024*1024)); //important for h2 download performance!
         httpc.enforce_http(false); // we want https...
 
         let https = HttpsConnector::with_connector(httpc, ssl_connector_builder.build());