blacklist shard_existing_coll_chunk_count.js from the continuous config stepdown suite

The config stepdown suite cannot handle tests doing count against the config database: count is not retryable. Such tests are blacklisted. shard_existing_coll_chunk_count.js does a count on a config server collection here – also, remove the 'getPrimary()' bit in v3.4 that was added to attempt to minimize the failure in the stepdown suite.