Remove unnecessary allow(dead_code) from testing example #3767

Job	Run time
cargo-hack	8m 45s
check-docs	53s
check	3m 19s
cargo-public-api-crates (axum)	1m 33s
cargo-public-api-crates (axum-core)	1m 9s
cargo-public-api-crates (axum-extra)	1m 28s
cargo-public-api-crates (axum-macros)	1m 7s
cargo-deny check (advisories)	1m 48s
cargo-deny check (bans licenses sources)	1m 42s
dependencies-are-sorted	1m 12s
test-msrv	6m 45s
test-nightly	4m 3s
armv5te-unknown-linux-musleabi	1m 20s
test-docs	4m 39s
wasm32-unknown-unknown	46s
test-versions (stable)	4m 28s
test-versions (beta)	8m 39s
	53m 36s

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