Workaround for missing migrate-deregister command in Drush 10.3.1?
Those migrations are stored as configuration.
When you run drush migrate-upgrade --configure-only the command created various migrate_plus.migration.* config data related to your D7 migration.
If you remove the config, you'll remove the migrations. For example:

If you have config captured in a sync directory, do a drush cex, remove all migrate_plus.migration.* yml files, then drush cim. Drush will drop all migration configuration on import.
If you'd rather delete the migration directly, drush cdel will let you specifically delete a migration.

Even if you delete the migration config, some migration metadata is still kept around in the database (see migrate_map_ &amp; migrate_messages_ tables). They'll be re-used if you re-import migration config. If you're finally finished with your migrations, you can delete them.