Remove `Gitlab::Git::Storage::Circuitbreaker` code

Since Gitaly is hitting 1.0.0 soon, after talking with @zj, I think we can start thinking about getting rid of this code.

It's currently only running in Canary: https://gitlab.com/gitlab-com/infrastructure/issues/3403

Things we could remove if we decide we don't need it anymore:

• lib/gitlab/git/storage/circuit_breaker.rb
• lib/api/circuit_breakers.rb
• The application settings for the circuitbreaker
• bin/storage_check: The executable that hit's an endpoint checking the shards
• The endpoint hit for the storage check
• lib/gitlab/storage_check
• The configuration in omnibus: omnibus-gitlab!2137 (merged)
• Circuitbreaker alerts in the runbooks: https://dev.gitlab.org/cookbooks/runbooks/blob/master/alerts/nfs-circuitbreaker-tripped.rules & https://dev.gitlab.org/cookbooks/runbooks/blob/master/alerts/nfs-circuitbreaker-slow.rules

I know we had planned to reuse some of the codes for other paths: https://gitlab.com/gitlab-org/gitlab-ce/issues/37227 But that's currently not being moved forward since we don't have results from production, and we're moving to object storage quite fast. We could always resurrect the parts we need, since it's all in the repo anyway 😄.

/cc @DouweM @jramsay