Remove protected paths throttle from Omnibus

From %12.4, protected paths throttle is now handled by GitLab Rails (gitlab#29952 (closed)), making the Omnibus throttle deprecated (#4688 (closed), !3597 (merged)).

From the Omnibus Deprecation policy

For regular configuration, removal target should always be the date of the next major release.

The protected paths throttle is scheduled to be removed on %13.0

Backend plan

• Remove rack_attack.rb.erb
• Remove GitLabRails#parse_rack_attack_protected_paths
• Remove Setting up paths to be protected by Rack Attack documentation
• Remove Setting up throttling for ‘paths to be protected’ documentation
• Remove it from attribute list
• Remove the related specs