Consolidate duplicate code paths for deleting users

[dnephin]

@j-sneh noticed this while working on #2767

Today we have the following:

• access.DeleteIdentity - handles removing credentials, access keys, and grants associated with the identity, and fix: remove deleted identities from identities_groups #2767 adds removal of identities_group references
• data.DeleteIdentity - only deletes the one row from identities table
• data.DeleteIdentities - handles removing grants, and fix: remove deleted identities from identities_groups #2767 adds removal of identities_group references
• Server.loadUsers (from server config) - calls data.DeleteIdentities but does not remove credentials, or access keys for the users
• access.DeleteProviders - calls data.DeleteIdentities and handles deleting access keys (doesn't need to delete credentials)

With #2767 merged it seems like the only gap may be loadUsers not deleting of credentials or access keys. I'm not sure if that is by design.

The problem now is that there are two ways to delete users, in one code path the logic to delete associations is handled in access, and in the other code path it's handled in data.

This issue is to clean this up so that at the very least the two flows are consistent about where associations are deleted, either access or data. Ideally we would also remove the need to have separate code paths for deleting one identity and deleting many identities, but I'm not sure if that's possible.

[stale]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?

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