Remove unused `mut` #3859

Job	Run time
cargo-hack	8m 8s
check	4m 8s
check-docs	31s
cargo-public-api-crates (axum)	1m 23s
cargo-public-api-crates (axum-core)	1m 26s
cargo-public-api-crates (axum-extra)	1m 38s
cargo-public-api-crates (axum-macros)	1m 30s
cargo-deny check (advisories)	34s
cargo-deny check (bans licenses sources)	38s
dependencies-are-sorted	1m 25s
Spell Check with Typos	6s
test-nightly	3m 38s
test-versions (stable)	5m 37s
test-versions (beta)	8m 32s
test-docs	4m 9s
armv5te-unknown-linux-musleabi	1m 26s
test-msrv	7m 27s
wasm32-unknown-unknown	25s
	52m 41s

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