Resolve "rollout feature flag REAL_NAME_SYSTEM"

related MR !416 (merged) and that MR was reverted at #924 (closed).

This MR is a clean up issue of the real_name_system feature flag. The clean up steps are documented at development docs.

What's been done in this issue

• remove real_name_system.yml
• update ::Feature.enabled?(:real_name_system) to SaaS Only for if statements on backend
• update gon.real_name_system = true to SaaS Only for gon_helper.rb

An extra step is needed for SRE to cleanup the database after releasing

/chatops run feature delete real_name_system --staging
/chatops run feature delete real_name_system

Steps of verification

1. Goto registration page
2. You should see the phone input is shown on that page
3. You should see your phone number on your profile page after registering