If a user is removed from credentials.users in config and the instance is reloaded, the user still exists in box.
This leaves stale users in the system, which is undesirable from a security perspective.

Example

Initial config:

credentials:
  users:
    alice:
      password: ALICE
    bob:
      password: BOB

After applying, both alice and bob exist:

box.schema.user.exists('alice') -- true
box.schema.user.exists('bob')   -- true

Then we update config and remove alice:

credentials:
  users:
    bob:
      password: BOB

Expected behavior after reload:

box.schema.user.exists('alice') -- false (user dropped)
box.schema.user.exists('bob')   -- true

Actual behavior:

box.schema.user.exists('alice') -- true  (stale user remains)
box.schema.user.exists('bob')   -- true

TNTP-2155