Fix Cassandra monthly loading application path

Since we added the last changes to the SparkSqlOperator, we uncovered a legacy line of code that we should have removed in the cassandra_monthly_load DAG. Namely, the application parameter should not be overridden when calling the SparkSqlOperator. This was correct in the hourly and daily Cassandra-loading counterpart DAGs, but for some reason it was wrong in the monthly one.