Dolt system functions ignore database qualifier.

This is best illustrated with an example:

```sql
CREATE DATABASE d1;
USE d1;
CALL dolt_checkout('-b', 'new');
CREATE DATABASE d2;
USE d2;
CALL dolt_checkout('-b', 'newer');
SELECT active_branch(); -- returns "newer"
USE d1;
SELECT active_branch(); -- returns "new"
SELECT d1.active_branch(); -- returns "new"
SELECT d2.active_branch(); -- returns "new"
SELECT does_not_exist.active_branch(); -- returns "new"
```

I'm not sure what the correct behavior should be when accessing system tables from another database, but the fact that the qualifier is parsed and then silently ignored feels wrong.