Removed numShardedCollections metric from serverStatus command output

Description of Linked Ticket

Due to the introduction of tracked unsharded collections (PM-3364) the numShardedCollections metric reported in server status is now wrong because it reports both sharded and tracked unsharded collections.

In order to report only sharded collection, we would need to perform a read on config.collections filtering out tracked unsharded collection. Unfortunately, this operation is to slow to be executed as part of serverStatus command execution. On the other hand maintaining a cached count of the sharded collection would require a significant effort.

Since the number of sharded collection can be easily retrieved by means of user facing server API we decided to remove this metric from the serverStatus command.