metalman: delete unreferenced helpers

[Philip Lombardi (plombardi89)]

Part of the #670 dead-code sweep, split by component. The tooling that produced these findings is #673; this PR is deletions only and is independent of the other eight — the file sets are disjoint, so they can be reviewed and merged in any order and in parallel.

Why two analyzers were needed to find this

unused cannot see dead exported code under internal/. That is a documented design decision, not a gap: unused/unused.go:48-62 holds that a package uses its exported named types and a named type uses its exported methods, so every method on an exported type is transitively live because the type is exported.

x/tools/cmd/deadcode does not close that gap on its own either. x/tools/go/callgraph/rta/rta.go:281-287,433-437 — converting a value to an interface materializes its runtime type and marks every exported method of that type reachable, because reflection could call any of them. One var i any = x anywhere buys all of x's exported methods a clean bill of health.

So #673 adds both deadcode and hack/cmd/unreferenced, which resolves identifiers instead of tracing reachability. Neither subsumes the other, and findings below are attributed to whichever one saw them.

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metalman: delete unreferenced helpers

netboot.OCICache loses five default-architecture wrappers - DigestFor,
ImageDir, IsCached, Metadata and InvalidateRef - each a one-liner delegating to
its ...ForArchitecture counterpart with v1alpha3.DefaultPXEArchitecture. The
architecture-aware variants are what the reconciler and the netboot handlers
call; the wrappers are what multi-architecture support left behind.
InvalidateRefForArchitecture goes with them: once InvalidateRef was gone it had
no caller of its own.

commands.PrintStep had no reference. The cyan constant it was the only user of
goes with it; the identically named constant in
cmd/kubectl-unbounded/app/machine_ops.go is live and untouched.

machineops' reasonWaitingForOlderOperation is an unexported constant with no
reader. It sits in a const group whose other members are all live, which is why
unused never reported it, and the string "WaitingForOlderOperation" does not
appear anywhere else in the tree either.

Refs #670

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Verification

go build ./..., go vet ./..., golangci-lint over the touched trees (0 issues), make fmt producing no diff, and go build -tags e2e,integrationtest,storageboundary ./.... Full suite runs in CI.

The eight deletion branches were reassembled onto the tooling commits and diffed against the original combined branch: byte-identical, so nothing was lost or duplicated in the split.