metrics, policy/v2: drop unused scaffolding + nil-error returns

Two cleanups in the same package boundary:

policy/v2.Policy.compileFilterRules and compileFilterRulesForNode
returned (rules, error) where the error was always nil. Drop the
error return and the dead error handling at 15 call sites in the
compat tests.

Delete code with no callers:
  hscontrol/metrics.go
    prometheusMiddleware + respWriterProm — custom HTTP timer
    middleware that was never registered. /metrics stays mounted
    via debug.go and noise.go; mapresponse_* counters keep emitting.
    httpDuration and httpCounter — backing metrics for the deleted
    middleware.
  hscontrol/policy/v2/filter.go
    resolvedAddrsToPrincipals — superseded by ipSetToPrincipals.
    ipSetToPrefixStringList — superseded by inline prefix
    formatting at the surviving callers.
  hscontrol/policy/v2/tailscale_{acl,grants}_data_compat_test.go
    setupACLCompatNodes / setupGrantsCompatNodes — the data-driven
    tests switched to per-scenario topology reconstruction.