[charliepark]

In working on a console PR (oxidecomputer/console#3211) I discovered that #8605 was still open. (#9276 handled deletion of role assignments for SCIM-managed users / groups, but not for local ones). This PR adds a method that handles the deletion of role assignments for locally-managed users and groups, and calls it when the user / group is deleted.

Note: there is no method to delete groups locally, but deleting a silo without addressing the roles would leave some orphaned rows, so this code cleans up the roles during the silo deletion process.

"But wait!", I hear you say, "what about the orphaned rows from users that have already been deleted from the silo? Isn't that what prompted #8605 in the first place?" Good question. That is outside the scope of this PR, but the robot suggests there's a migration from before that covers similar territory: schema/crdb/cleanup-orphaned-subnet-pool-silo-links (version 248):

SET LOCAL disallow_full_table_scans = off;

DELETE FROM omicron.public.subnet_pool_silo_link
WHERE subnet_pool_id NOT IN (
    SELECT id FROM omicron.public.subnet_pool WHERE time_deleted IS NULL
);

So that's something we can talk through separately.

Unrelated, but I also came across some old uncalled code in silo_group_delete; this PR removes that method as well.

Closes #8605