Remove outdated reference to PopArgumentAsync #1016

Job	Run time
Rustfmt	14s
Clippy	50s
Test (MSRV)	1m 27s
Build docs	2m 38s
Test (stable)	2m 3s
Test (beta)	2m 2s
Test (nightly)	2m 0s
Test (macOS)	3m 38s
Test (Windows)	3m 59s
Test (no features)	1m 8s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 13s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 20s
	22m 32s

Provide feedback