chore: Remove unused mutability #4042

Job	Run time
clippy	41s
codegen	16s
deny	8s
doc-test	1m 8s
external-types	30s
check (ubuntu-latest)	2m 4s
check (macOS-latest)	1m 52s
Interop Tests (ubuntu-latest)	26s
msrv	39s
Interop Tests (macOS-latest)	41s
check (windows-latest)	3m 44s
rustfmt	6s
Interop Tests (windows-latest)	3m 41s
semver	1m 7s
udeps	3m 45s
test (ubuntu-latest)	1m 22s
test (macOS-latest)	1m 45s
test (windows-latest)	4m 16s
	28m 11s

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