Don't error when trying to delete md5 generated username from mysql

This was causing the tests to fail because the unbind operation returns an error, even though the broker successfully deleted the actual user that was created.

Bindings created after circa 2018 all use the new SHA256 method of username generation, and it's fine if the MD5 generated username doesn't exist, which it almost never will.

sqlengine/mysql_engine.go

@@ -186,7 +186,7 @@ func (d *MySQLEngine) DropUser(bindingID string) error {
 		return err
 	}
 
-	dropUserStatement = "DROP USER `" + username + "`@`%`;"
+	dropUserStatement = "DROP USER IF EXISTS`" + username + "`@`%`;"
 	logger.Debug("drop-user", lager.Data{"statement": dropUserStatement})
 
 	_, err = d.db.Exec(dropUserStatement)