sql: prevent users from being dropped if they have default privileges defined for

[jseldess]

PR: cockroachdb/cockroach#67950

From release notes:

If a user has a default privilege defined for, they cannot be dropped until the default privilege is removed. Example: ALTER DEFAULT PRIVILEGES FOR ROLE test1 GRANT SELECT ON TABLES TO test2;. Neither test1 nor test2 can be dropped until performing ALTER DEFAULT PRIVILEGES FOR ROLE test1 REVOKE SELECT ON TABLES FROM test2;. [#67950][#67950]