Cannot rollback data that goes into user_data destination

Problem/Motivation

Rolling back a migration that has a destination of "user_data" does not work.

Steps to reproduce

Create a custom migration that has a destination of "user_data".
Migrate the data.
Confirm the data was migrated correctly.
Rollback the migration using the command "drush migrate:rollback".

Expected outcome:
* The migrated data is removed.

Actual outcome:
* The migrated data is still present in the database.

Proposed resolution

Allow data that is migrated into user_data to be removed when the "drush migrate:rollback" command is executed in Migrate Tools.

Remaining tasks

Work out the cause of the problem.

User interface changes

Rolling back a migration to the user_data table deletes the data.

API changes

TBD

Data model changes

TBD

Release notes snippet

TBD