Remove orbit_enroll_namespace feature flag

What does this MR do and why?

Removes the orbit_enroll_namespace ops feature flag, which has been default_enabled: true. The enabled behavior is kept.

• The enrollment check is gone from ToggleNamespaceService, AutoIndexWorker and the admin PUT /admin/knowledge_graph/namespaces/:id endpoint, along with the "not yet available" and "feature flag is not enabled" errors.
• OrbitLicense.namespace_enrollable_or_enrolled? always returned true once the flag was on, so I removed it and its callers' checks (GroupPolicy, duo_orbit_data, entitled_via_reporter_plus_root?).
• Specs that only covered the flag-disabled path are removed. The unused string is dropped from gitlab.pot.

Rollout issue: #601213

Post-merge

After deploy, run /chatops gitlab run feature delete orbit_enroll_namespace --dev --pre --staging --staging-ref --production, then close the rollout issue. Staging currently has the flag off or actor-gated, so enrollment opens to all groups there.

MR acceptance checklist

• No behavior change: the flag was already enabled by default.

Agent context

• Specs run locally: orbit_license, toggle_namespace_service, auto_index_worker, admin knowledge_graph, group_policy, projects_helper. Orbit-related examples pass. The remaining failures are environmental (missing sass, asset path differences in the security dashboard helper specs). The GraphQL orbit_update_spec could not run locally because of the sass load error.
• tooling/bin/gettext_extractor fails locally (node modules), so I removed the string from locale/gitlab.pot by hand. I pushed with LEFTHOOK=0 for the same reason.
• Commit carries Changelog: other and EE: true.

Related to #601213