Remove TransferDomainRequest and references

[nospame]

Technical Summary

https://dimagi.atlassian.net/browse/SAAS-19359
This removes the TransferDomainRequest model, as no live projects have completed domain transfer records on dimagi environments. Live code referencing this model was removed in #37346

Safety Assurance

Safety story

This is straightforward as model deletions go - no other models or still-existing code depend on this model, and previous code was feature flagged. Ensuring #37346 is deployed first makes sure there are no unexpected errors because of references to this model when the migration is run on a live environment.

Automated test coverage

Test cases for the removed TransferDomainRequest have been removed. Tests for domain dump and deletion that look at related models still pass as the references to this model have been removed.

QA Plan

Not planning it.

Migrations

• The migrations in this code can be safely applied first independently of the code. Pay particular attention to backward incompatible operations like RemoveField, RenameField, RemoveConstraint, and others described here that can cause errors when migrations are applied to a live database.

Rollback instructions

• This PR can be reverted after deploy with no further considerations
  The migration here would first have to be reversed.

Labels & Review

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change

The base branch was changed.