Refactor fetching healthy shards with Gitlab::HealthChecks::GitalyCheck
Refactor fetching healthy shards with Gitlab::HealthChecks::GitalyCheck

There is only 1 `HEALTHY_SHARD_CHECKS` used:
Gitlab::HealthChecks::GitalyCheck

So we can simplify code to get the list of healthy shard names.

axilleas/gitlabhq,jirutka/gitlabhq,axilleas/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,axilleas/gitlabhq,mmkassem/gitlabhq,dreampet/gitlab,mmkassem/gitlabhq,jirutka/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,iiet/iiet-git,dreampet/gitlab,dreampet/gitlab,stoplightio/gitlabhq,dreampet/gitlab,mmkassem/gitlabhq,jirutka/gitlabhq