mssql: Exclude dbo schema from drop schema statements

When using the CLI "destroy" command to expire an environment, exclude the MSSQL built in 'dbo' schema from DROP SCHEMA statements.

Error:
```
Error: Failed to drop the expired environment schema 'Database.dbo': (15150, b"Cannot drop the schema 'dbo'.DB-Lib error message 20018, severity 16:\nGeneral SQL Server error: Check messages from the SQL Server\n")
```