Fix upload_lots for 8.4.0

This test is hanging on Linux with the 8.4.0 update.

My poor understanding is that `perform()` should not be used with
`curl_multi_socket_action` (that is, it should only use `perform` or
`action`, not both). Removing this seems to fix things.

Note that there were some other hang-related changes in the 8.4.0
release. I bisected the hang to curl/curl#11939,
which then uncovered some curl tests that were hanging in
curl/curl#12033. That then spawned off
two more changes to fix the hangs (both of which are also in 8.4.0).
However, this upload_lots was still failing. I think this is likely
due to an incorrect use of the API poorly interacting with the timer
changes in 8.4.0.

tests/multi.rs

@@ -140,7 +140,6 @@ fn upload_lots() {
 
     let e = t!(m.add(h));
 
-    assert!(t!(m.perform()) > 0);
     let mut next_token = 1;
     let mut token_map = HashMap::new();
     let mut cur_timeout = None;