Remove WiredTiger specific upgrade/downgrade interface from kv engine layer.

SERVER-32825 added an API alterIdentMetadata in kv engine to enable unique index metadata update in WiredTiger storage engine. This is a layer violation since it is a method related to WT internal concept in non-storage-engine-specific interface. Currently this API is required to upgrade WiredTiger unique indexes. 

In a future release, when all WiredTiger unique indexes are upgraded, this interface can be removed.