Remove sync_audit_events_to_clickhouse feature flag

What does this MR do and why?

Remove sync_audit_events_to_clickhouse feature flag

For now we are splitting table of audit events into 4 new tables to support cells architecture, so clickhouse migration is blocked therefore we can remove the feature flag along with the code behind it

Changelog: removed

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Before	After

Migration Logs

bundle exec rake db:migrate:up:main VERSION=20241224062532
DEPRECATION WARNING: Support for Rails versions < 7.1 is deprecated and will be removed from ViewComponent 4.0.0 (ViewComponent v4 will remove support for Rails versions < 7.1 no earlier than April 1, 2025) (called from <main> at /Users/harsimar/dev/gitlab-development-kit/gitlab/config/environment.rb:7)
main: == [advisory_lock_connection] object_id: 130280, pg_backend_pid: 73205
main: == 20241224062532 RemoveAuditEventSyncWorkerJobs: migrating ===================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: == 20241224062532 RemoveAuditEventSyncWorkerJobs: migrated (0.0045s) ==========

main: == [advisory_lock_connection] object_id: 130280, pg_backend_pid: 73205

bundle exec rake db:migrate:down:main VERSION=20241224062532
DEPRECATION WARNING: Support for Rails versions < 7.1 is deprecated and will be removed from ViewComponent 4.0.0 (ViewComponent v4 will remove support for Rails versions < 7.1 no earlier than April 1, 2025) (called from <main> at /Users/harsimar/dev/gitlab-development-kit/gitlab/config/environment.rb:7)
main: == [advisory_lock_connection] object_id: 130280, pg_backend_pid: 73827
main: == 20241224062532 RemoveAuditEventSyncWorkerJobs: reverting ===================
main: == 20241224062532 RemoveAuditEventSyncWorkerJobs: reverted (0.0040s) ==========

main: == [advisory_lock_connection] object_id: 130280, pg_backend_pid: 73827

Related to #427738