Remove duplicate index on cluster_agents

What does this MR do?

Removes a redundant cluster_agents index on project_id

    "index_cluster_agents_on_project_id_and_name" UNIQUE, btree (project_id, name)
    "index_cluster_agents_on_project_id" btree (project_id)

Output

== 20200921203231 RemoveDuplicateClusterAgentsIndex: reverting ================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:cluster_agents, :project_id, {:name=>"index_cluster_agents_on_project_id", :algorithm=>:concurrently})
   -> 0.0030s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- add_index(:cluster_agents, :project_id, {:name=>"index_cluster_agents_on_project_id", :algorithm=>:concurrently})
   -> 0.0047s
-- execute("RESET ALL")
   -> 0.0003s
== 20200921203231 RemoveDuplicateClusterAgentsIndex: reverted (0.0087s) =======

== 20200921203231 RemoveDuplicateClusterAgentsIndex: migrating ================
-- transaction_open?()
   -> 0.0000s
-- indexes(:cluster_agents)
   -> 0.0033s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- remove_index(:cluster_agents, {:algorithm=>:concurrently, :name=>"index_cluster_agents_on_project_id"})
   -> 0.0035s
-- execute("RESET ALL")
   -> 0.0002s
== 20200921203231 RemoveDuplicateClusterAgentsIndex: migrated (0.0078s) =======

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

#244484 (closed)