chore(nns): Clean up the migration code to backfill InstallCode hashes #25841

Job	Run time
Bazel Test All	12m 26s
Bazel Build All Config Check	0s
Bazel Build Fuzzers	1m 24s
Bazel Build Fuzzers AFL	2m 30s
Bazel Test macOS Intel	0s
Build IC	5m 52s
Cargo Clippy Linux	3m 40s
Python CI Tests	1m 45s
Cargo Build Release Linux	7m 55s
Build Determinism (//publish/binaries:upload, release, build-ic/release, false)	16s
Build Determinism (//publish/canisters:upload, canisters, build-ic/canisters, false)	13s
Build Determinism (//ic-os/guestos/envs/prod:upload_disk-img, guest-os/update-img, build-ic/icos/...	13s
Build Determinism (//ic-os/hostos/envs/prod:upload_update-img, host-os/update-img, build-ic/icos/...	13s
Build Determinism (//ic-os/setupos/envs/prod:upload_disk-img, setup-os/disk-img, build-ic/icos/se...	18s
	36m 45s

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