Disable PeriodicShardedIndexConsistencyChecker for cursor_valid_after_shard_stepdown.js

cursor_valid_after_shard_stepdown.js expects the the shard version after step down to be UNKNOWN. 

However, the test doesn't account for a potential shard metadata refresh caused by the PeriodicShardedIndexConsistencyChecker and can fail.

The PeriodicShardedIndexConsistencyChecker should be disabled for this test.