Dropping database in RDS follows different method than dropping the database in local SQL Server.
Below is the command to DROP the database in Amazon RDS
EXECUTE msdb.dbo.rds_drop_database N'demodb'
However when the SQL Server is configured with Multi-zone mirroring, then you need to set the partner to OFF and drop all active connections and then rollback any active sessions and drop the database.
FYI - Please make sure you test the script in DEV / lower environment before you implement on actual live database.