Remove CircleCI configuration

[jaredlockhart]

Problem

All CircleCI jobs have been migrated to GHA:

• Check workflows (Experimenter, Cirrus, Schemas, Feature Manifests)
• Integration tests (Nimbus UI, Enrollment, Targeting, Remote Settings)
• Deploy jobs (Experimenter, Cirrus, Schemas)
• Update/cron jobs (External Configs, Application Services, Firefox Versions)

The .circleci/config.yml still contains the old job definitions (now running in parallel with GHA). Once we're confident GHA is stable, CircleCI can be fully removed.

Remaining in CircleCI

• Mobile integration tests (Fenix, iOS enrollment) — broken independently, not ported
• build_firefox_fenix — part of mobile pipeline
• create_mobile_recipes — part of mobile pipeline
• setup-github-bot command — unused after cron job migration
• docker_login command — unused after Cirrus deploy migration

Acceptance criteria

• Verify all GHA workflows are stable for 1-2 weeks
• Remove .circleci/config.yml (or reduce to mobile-only if keeping those)
• Disable CircleCI integration on the repo

┆Issue is synchronized with this Jira Task