We can use delete_db_instance on rds client as given below:
rds_client = boto3.client('rds')
for cluster in result ["rds"]:
print ("RDS DB is about to delete %s | in %s" % (cluster['DBInstanceIdentifier']))
response = rds_client.delete_db_instance(DBInstanceIdentifier=cluster['DBInstanceIdentifier'])
SkipFinalSnapshot=True
DeleteAutomatedBackups=True