Before this change, `saslauthd_authenticator` prevented dropping
roles. The current documentation instructs users to `Ensure Scylla has
the same users and roles as listed in the LDAP directory`. Therefore,
ScyllaDB should allow dropping roles so administrators can remove
obsolete roles from both LDAP and ScyllaDB.
The code change is minimal — dropping a role is a no-op, similar to the
existing no-op implementations for successful `create` and `alter`
operations.
`saslauthd_authenticator_test` is updated to verify that dropping
a role doesn't throw anymore.
Fixes: scylladb/scylladb#25571Closesscylladb/scylladb#25574
86fc513bd9