Remove deprecated `lightning::util::string` re-exports #19100

Job	Run time
ext-test	1m 13s
build (windows-latest, stable)	1m 13s
build (ubuntu-latest, stable)	1m 14s
build (ubuntu-latest, 1.63.0)	1m 13s
build (ubuntu-latest, beta)	1m 16s
build (macos-latest, stable)	1m 52s
build (macos-latest, beta)	1m 19s
build (windows-latest, beta)	1m 13s
linting	1m 13s
build (macos-latest, 1.63.0)	1m 19s
rustfmt	15s
coverage	1m 16s
incremental-mutants	1m 18s
check_commits	1m 13s
benchmark	1m 16s
check_release	1m 15s
build-tx-sync (ubuntu-latest, beta)	1m 18s
build-tx-sync (ubuntu-latest, stable)	1m 13s
build-tx-sync (ubuntu-latest, 1.75.0)	1s
build-tx-sync (macos-latest, beta)	1s
build-tx-sync (macos-latest, stable)	1s
build-tx-sync (macos-latest, 1.75.0)	1m 4s
fuzz	1s
	23m 17s

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