Remove tmp_index_approval_project_rules_scanners

What does this MR do?

Remove tmp_index_approval_project_rules_scanners from approval_project_rules table. It was added only to support a pos-deployment migration.

Related issue: #337776 (closed)

Before migration

Indexes:
    "approval_project_rules_pkey" PRIMARY KEY, btree (id)
    "any_approver_project_rule_type_unique_index" UNIQUE, btree (project_id) WHERE rule_type = 3
    "index_approval_project_rules_on_id_with_regular_type" btree (id) WHERE rule_type = 0
    "index_approval_project_rules_on_project_id" btree (project_id)
    "index_approval_project_rules_on_rule_type" btree (rule_type)
    "tmp_index_approval_project_rules_scanners" gin (scanners) WHERE scanners @> '{cluster_image_scanning}'::text[]

the Migration

$ bundle exec rails db:migrate
== 20210819152723 RemoveTmpIndexApprovalProjectRulesScanners: migrating =======
-- transaction_open?()
   -> 0.0000s
-- indexes(:approval_project_rules)
   -> 0.0077s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:approval_project_rules, {:algorithm=>:concurrently, :name=>"tmp_index_approval_project_rules_scanners"})
   -> 0.0043s
-- execute("RESET statement_timeout")
   -> 0.0005s
== 20210819152723 RemoveTmpIndexApprovalProjectRulesScanners: migrated (0.0148s)

After the Migration

Indexes:
    "approval_project_rules_pkey" PRIMARY KEY, btree (id)
    "any_approver_project_rule_type_unique_index" UNIQUE, btree (project_id) WHERE rule_type = 3
    "index_approval_project_rules_on_id_with_regular_type" btree (id) WHERE rule_type = 0
    "index_approval_project_rules_on_project_id" btree (project_id)
    "index_approval_project_rules_on_rule_type" btree (rule_type)

the Rollback

$ bundle exec rails db:rollback
== 20210819152723 RemoveTmpIndexApprovalProjectRulesScanners: reverting =======
== 20210819152723 RemoveTmpIndexApprovalProjectRulesScanners: reverted (0.0000s)

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Does this MR meet the acceptance criteria?

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