Clean up old Feature Flags

I have gathered just some of the feature flags that have been defaulted on for long enough for us to safely remove them. While these should certainly be removed in separate MRs, I did not feel like they all deserved separate issues.

• :set_user_last_activity defaulted on since 2019-02-05
• :registrations_recaptcha defaulted on since 2019-06-25
• :attempt_group_search_optimizations defaulted on since 2019-02-28
• :attempt_project_search_optimizations defaulted on since 2019-05-21
• :emails_disabled defaulted on since 2019-08-17
• :create_confidential_merge_request defaulted on since 2019-07-05
• :project_list_show_mr_count defaulted on since 2018-12-19
• :project_list_show_issue_count defaulted on since 2018-12-19
• :application_settings_tokens_optional_encryption defaulted on since 2019-03-18
• :ci_dag_support defaulted on since 2019-08-14
• :ci_pipeline_persisted_stages defaulted on since 2019-02-18
• :ci_runners_tokens_optional_encryption defaulted on since 2019-03-18
• :reactive_caching_check_key_exists defaulted on since 2019-07-08

/cc @rpaik this would be great for community contributors