Fix flaky test - dtest.secondary_indexes_test.TestSecondaryIndexes.test_6924_dropping_ks

Failed 1 times in the last 30 runs. Flakiness: 6%, Stability: 96%
Error Message

cassandra.OperationTimedOut: errors=

{'127.0.0.2:9042': 'Client request timeout. See Session.execute[_async](timeout)'}

, last_host=127.0.0.2:9042