chore(swarm-tests): remove async-std feature

Amendment to #6064.
#6064 removed the `async-std`-based variant for creating a test-swarm, so the `async-std` feature is now not needed anymore.

Pull-Request: #6068.

Author: elenaf9

swarm-test/Cargo.toml

@@ -26,7 +26,6 @@ futures-timer = "3.0.3"
 [features]
 default = ["tokio"]
 
-async-std = ["libp2p-swarm/async-std", "libp2p-tcp/async-io"]
 tokio = ["libp2p-swarm/tokio", "libp2p-tcp/tokio"]
 
 [lints]