hack: delete unreferenced dev-tooling 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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hack: delete unreferenced dev-tooling helpers

Zero-reference removals in the development tooling:

• notice/internal/npm.WithFS. The identically named gomod.WithFS is used by
  gomod's tests and stays.
• notice/internal/testutil.BSD3License. MITLicense, Apache2License and
  WriteTree in the same file are all used.
• relctl/version.GitRepo.Dir.
• relctl/gh.Client.API, .Name and .Owner. These had callers - three lines in
  client_test.go - but the test is in package gh, so it can read api, owner
  and repo directly. Exported accessors that exist only for a same-package
  test are surface with no reader.
• relctl's actionsError, an unexported constant in a const group whose other
  members are live, which is why unused never reported it.

orcadev:

• hostPortFromURL had no caller outside its own test; both go.
• percentile sorted its input and delegated to percentileSorted. Only its
  test called it; computeLatencyStats, the live path, calls percentileSorted
  directly having sorted once. The wrapper goes and TestPercentile moves to
  percentileSorted with pre-sorted input, which keeps the p0-to-min and
  p100-to-max edge cases that computeLatencyStats' own test does not cover.

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.