can't fully remove a database from user grants

### Feature Request

__Proposal:__ [Description of the feature]

Dropping a database should remove it from all user grants.

__Current behavior:__ [What currently happens]

DBs are listed forever, even if no privs and the db is dropped.

__Use case:__ [Why is this important (helps with prioritizing requests)]

Cleanliness, esp if many databases are created over time.

```
> show grants for alice
database privilege
boop READ

> drop database boop
> show grants for alice
database privilege
boop READ

> revoke all on boop from alice
> show grants for alice
database privilege
boop NO PRIVILEGES

> grant read on mydb to alice
> show grants for alice
database privilege
boop NO PRIVILEGES
mydb READ

> revoke all on mydb from alice
> show grants for alice
database privilege
boop NO PRIVILEGES
mydb NO PRIVILEGES

>
```

Reproduce the issue with the shown grant, drop database, show grants, and revoke commands. Trace the database-drop and user-grant handling paths to find where database entries remain. Done means dropping a database removes it from every user's grants, including entries with no remaining privileges, and the example sequence no longer lists it.

Written by the indexing model from the issue text.