release: drop activator release pipeline

[elitegreg]

Summary

• Delete every build/release path that produces a doublezero-activator artifact: release.activator.yml, release.devnet.activator.daily.yml, the three .goreleaser.*.activator.yaml files, and the doublezero-activator.service systemd unit.
• Remove activator matrix entries from release.testnet.push.tags.components.yml, release.devnet.all.daily.yml, and release.pipeline.validation.yml so a push to main no longer produces activator artifacts in any environment.

Phase 1.2 of the activator removal effort (tracker #3607). The activator crate stays for now (deleted in Phase 3, #3612) so local cargo build --workspace still produces the binary for development. Cloudsmith retention is unchanged.

E2E workflow references to the activator container (e2e.yml, shreds-e2e.yml) are intentionally left in place — the tracker assigns them to Phase 2 (#3609 / #3610 / #3611).

Gate: infra PR malbeclabs/infra#1175 (stop deploying the activator) must merge before this one. Otherwise the still-active nightly deploy workflow runs, fails to find new artifacts, and pages someone.

Closes #3608

Testing Verification

• grep -rni activator .github/workflows/release release/ returns nothing.
• Pipeline validation in this PR runs without the dropped activator matrix entry.