Remove code related to object hierarchy and MySQL

Closes #65056 and #65055

See merge request gitlab-org/gitlab-ce!31095

Convert several occurrences of `map` + `flatten` to
`flat_map` where applicable.

Respect alt email when sending group access notifications

See merge request gitlab-org/gitlab-ce!31089

When sending access granted/rejected emails we should also respect
custom emails set for groups/sub-groups

Support title and desc on merge w/ push option

See merge request gitlab-org/gitlab-ce!31068

These are not required because MySQL is not
supported anymore

MergeRequests::PushOptionsHandlerService has been updated to allow
creating and updating merge requests with the `title` and
`description` set using git push options.

To create a new merge request and set its title and description:

      git push -u origin -o merge_request.create \
        -o merge_request.title="My title" \
        -o merge_request.description="My description"

To update an existing merge request and set its title and
description:

      git push -u origin -o merge_request.title="My title" \
        -o merge_request.description="My description"

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/64320

Extra logging for new live trace architecture

See merge request gitlab-org/gitlab-ce!30892

Make pipeline emails respect group email setting

Closes #63485

See merge request gitlab-org/gitlab-ce!30907

Often live traces are removed even though the archived trace
doesn't exist. This commit checkes the existence strictly.