MLS: Backend sends remove proposal upon user deletion

[smatting]

Send external remove proposals when users are deleted.

Tracked by https://wearezeta.atlassian.net/browse/FS-906.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If this PR changes development workflow or dependencies, they have been A) automated and B) documented under docs/developer/. All efforts have been taken to minimize development setup breakage or slowdown for co-workers.
• If a cassandra schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• I updated changelog.d subsections with one or more entries with the following bits of information (details):