Remove subtransactions from `MergeRequest::Metrics` model

In #338346 (comment 652623314) and #338865 (comment 655312474), we identified the use of subtransactions can cause significant performance issues on PostgreSQL replicas at scale. If a long running transaction occurs, the subtransaction cache may not fit into the working set of memory, which can lead to a context switch storm when PostgreSQL needs to load subtransaction data from disk. While this arguably is a PostgreSQL bug, we should remove subtransactions entirely because they aren't performant at GitLab.com scale, nor are they strictly necessary. More context in https://gitlab.slack.com/archives/C02BHCTH02D/p1629392215352100?thread_ts=1629388693.342800&cid=C02BHCTH02D.

From https://thanos.gitlab.net/classic/graph?g0.range_input=2h&g0.stacked=0&g0.max_source_resolution=0s&g0.expr=sum(rate(gitlab_active_record_subtransactions_total%7Benv%3D%22gprd%22%7D%5B1m%5D))%20by%20(model)&g0.tab=0 you can see MergeRequest::Metrics is generating subtransactions. These should be removed from the codebase asap.

savepoint_backtraces

NOTE: @ahegyi believes this information is probably outdated since an MR from @engwan, which touched this code but may not have solved for this particular subtransaction.

This happened in https://log.gprd.gitlab.net/goto/8e3805df10714fff1bfdf3cdccf0789f:

"json": {
      "stage": "cny",
      "correlation_id": "01FE4KT6ANPWC8NEHKF05XZVF0",
      "savepoint_backtraces": [
        [
          "lib/gitlab/database/transaction/observer.rb:41:in `block in register!'",
          "app/models/application_record.rb:75:in `block in safe_find_or_create_by'",
          "lib/gitlab/database.rb:225:in `block in transaction'",
          "lib/gitlab/database.rb:224:in `transaction'",
          "app/models/application_record.rb:75:in `safe_find_or_create_by'",
          "app/models/merge_request.rb:1859:in `ensure_metrics'",
          "ee/lib/analytics/merge_request_metrics_refresh.rb:11:in `block in execute'",
          "ee/lib/analytics/merge_request_metrics_refresh.rb:10:in `each'",
          "ee/lib/analytics/merge_request_metrics_refresh.rb:10:in `execute'",
          "ee/app/services/ee/merge_requests/approval_service.rb:57:in `calculate_approvals_metrics'",
          "ee/app/services/ee/merge_requests/approval_service.rb:45:in `block in create_event'",
          "lib/gitlab/database.rb:225:in `block in transaction'",
          "lib/gitlab/database.rb:224:in `transaction'",
          "ee/app/services/ee/merge_requests/approval_service.rb:43:in `create_event'",
          "app/services/merge_requests/approval_service.rb:13:in `execute'",
          "ee/app/services/ee/merge_requests/approval_service.rb:14:in `execute'",
          "lib/api/merge_request_approvals.rb:58:in `block (3 levels) in <class:MergeRequestApprovals>'",
          "ee/lib/gitlab/ip_address_state.rb:10:in `with'",
          "lib/api/api_guard.rb:213:in `call'",
          "ee/lib/omni_auth/strategies/group_saml.rb:41:in `other_phase'",
          "lib/gitlab/jira/middleware.rb:19:in `call'"
        ]
      ],
      "level": "info",
      "type": "api",
      "component": "gitlab",
      "subcomponent": "application_json",
      "environment": "gprd",
      "tag": "api-cny-rails.var.log.containers.gitlab-cny-webservice-api-94fb4d79d-44wbg_gitlab-cny_webservice-b8c53fb522a894ab70a95fe313c4a77dccc3943cec14b06666f842bc7ff76571.log",
      "tier": "sv",
      "duration_s": 0.044940201987628825,
      "class": "Gitlab::Database::Transaction::Context",
      "savepoints_count": 1,
      "time": "2021-08-27T20:11:28.139Z",
      "result": "commit",
      "releases_count": 1,
      "rollbacks_count": 0,
      "shard": "default",
      "depth": 2,
      "sql": "/*application:web,correlation_id:01FE4KT6ANPWC8NEHKF05XZVF0,endpoint_id:POST /api/:version/projects/:id/merge_requests/:merge_request_iid/approve*/ BEGIN",
      "severity": "INFO"
    }