Remove `:approval_rule_name_index_for_code_owners` index

What does this MR do?

Remove :approval_rule_name_index_for_code_owners index from :approval_merge_request_rules table as we're ignoring the column in %13.3 (!37260 (merged)) and due to remove it entirely in %13.5 (#230835 (closed))

The down action is the same code as was used to create the index originally in fb4ad51b

Related to #221258 (closed) and #11834 (closed)

== 20200809171943 DropApprovalRuleNameIndexForCodeOwnersIndex: migrating ======
-- transaction_open?()
   -> 0.0000s
-- indexes(:approval_merge_request_rules)
   -> 0.0063s
-- remove_index(:approval_merge_request_rules, {:algorithm=>:concurrently, :name=>:approval_rule_name_index_for_code_owners})
   -> 0.0051s
== 20200809171943 DropApprovalRuleNameIndexForCodeOwnersIndex: migrated (0.0120s)

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

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• 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
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