Remove legacy index `index_approval_rule_name_for_code_owners_rule_type`

Once we've re-synced rule_type=2 approval rules, we can drop the legacy indices that do not take section into account.

LEGACY_INDEX_NAME_RULE_TYPE   = "index_approval_rule_name_for_code_owners_rule_type"
LEGACY_INDEX_NAME_CODE_OWNERS = "approval_rule_name_index_for_code_owners"

def up
  remove_concurrent_index_by_name :approval_merge_request_rules, LEGACY_INDEX_NAME_RULE_TYPE
  remove_concurrent_index_by_name :approval_merge_request_rules, LEGACY_INDEX_NAME_CODE_OWNERS
end

NOTE This migration likely isn't reversible, as the rules will be covered by a more specific unique index (added in !33121 (merged))