Remove partitionRacks from TopicMetadata

The ModernGroup#subscribedTopicMetadata takes too much memory due to 
partitionRacks:

/**

Map of every partition Id to a set of its rack Ids, if they exist.

If rack information is unavailable for all partitions, this is an empty map.
*/
private final Map<Integer, Set<String>> partitionRacks;
 
This is not being used at the moment as the consumer protocol does not support rack aware assignments.
 
A heap dump from a group with 500 members, 2K subscribed topic partitions shows
654,400 bytes used for partitionRacks. The rest of the ConsumerGroup object holds 822,860 bytes.