Don't require Gitlab::Redis in mail_room.yml if it's already defined

What does this MR do?

Avoid requiring lib/gitlab/redis.rb if Gitlab::Redis is already defined.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

Because otherwise you get already initialized constant Gitlab::Redis::XXX, e.g.:

› bin/rspec spec/config/mail_room_spec.rb
Running via Spring preloader in process 24658
/Users/remy/Code/GitLab/gdk/gitlab/lib/gitlab/redis.rb:3: warning: already initialized constant Gitlab::Redis::CACHE_NAMESPACE 
/Users/remy/Code/GitLab/gdk/gitlab/lib/gitlab/redis.rb:3: warning: previous definition of CACHE_NAMESPACE was here
/Users/remy/Code/GitLab/gdk/gitlab/lib/gitlab/redis.rb:4: warning: already initialized constant Gitlab::Redis::SESSION_NAMESPACE
/Users/remy/Code/GitLab/gdk/gitlab/lib/gitlab/redis.rb:4: warning: previous definition of SESSION_NAMESPACE was here
/Users/remy/Code/GitLab/gdk/gitlab/lib/gitlab/redis.rb:5: warning: already initialized constant Gitlab::Redis::SIDEKIQ_NAMESPACE
/Users/remy/Code/GitLab/gdk/gitlab/lib/gitlab/redis.rb:5: warning: previous definition of SIDEKIQ_NAMESPACE was here
/Users/remy/Code/GitLab/gdk/gitlab/lib/gitlab/redis.rb:12: warning: already initialized constant Gitlab::Redis::URL_MUTEX
/Users/remy/Code/GitLab/gdk/gitlab/lib/gitlab/redis.rb:12: warning: previous definition of URL_MUTEX was here
/Users/remy/Code/GitLab/gdk/gitlab/lib/gitlab/redis.rb:13: warning: already initialized constant Gitlab::Redis::POOL_MUTEX
/Users/remy/Code/GitLab/gdk/gitlab/lib/gitlab/redis.rb:13: warning: previous definition of POOL_MUTEX was here
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2 examples, 0 failures

What are the relevant issue numbers?

None!

Does this MR meet the acceptance criteria?

• CHANGELOG entry added Not needed.
• Documentation created/updated Not needed.
• API support added. Not needed.
• Tests. Not needed.
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together