Remove inconsistent index

What does this MR do?

This index was committed to the codebase on a backup table. It resulted in the index on gitlab.com not being consistent with the structure.sql entry in the codebase.

This table will be removed with #227753 (closed), as it follows a complex data migration regarding deduplication of labels. It's good to have the actual database match our schema though 😅

Related to #296756 (closed)

Migrations

$ bundle exec rake db:migrate:up VERSION=20210203002331
== 20210203002331 DropBackupLabelIndex: migrating =============================
-- transaction_open?()
   -> 0.0000s
-- indexes(:backup_labels)
   -> 0.0033s
-- execute("SET statement_timeout TO 0")
   -> 0.0001s
-- remove_index(:backup_labels, {:algorithm=>:concurrently, :name=>"backup_labels_project_id_title_idx"})
   -> 0.0053s
-- execute("RESET ALL")
   -> 0.0002s
== 20210203002331 DropBackupLabelIndex: migrated (0.0092s) ====================

$ bundle exec rake db:migrate:down VERSION=20210203002331
== 20210203002331 DropBackupLabelIndex: reverting =============================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:backup_labels, [:project_id, :title], {:name=>"backup_labels_project_id_title_idx", :unique=>true, :where=>"group_id = NULL::integer", :algorithm=>:concurrently})
   -> 0.0026s
-- execute("SET statement_timeout TO 0")
   -> 0.0001s
-- add_index(:backup_labels, [:project_id, :title], {:name=>"backup_labels_project_id_title_idx", :unique=>true, :where=>"group_id = NULL::integer", :algorithm=>:concurrently})
   -> 0.0161s
-- execute("RESET ALL")
   -> 0.0003s
== 20210203002331 DropBackupLabelIndex: reverted (0.0194s) ====================

Screenshots (strongly suggested)

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

Related to #296756 (closed)