SERVER-15700 Modify SortedDataInterface to not require unindex() to return whether the index row was deleted

Discussion: https://groups.google.com/forum/#!topic/mongodb-dev/75ZSCUrgmtk
Jira: https://jira.mongodb.org/browse/SERVER-15700

- SortedDataInterface::unindex() modified to return void, no longer checks for existence of target index key
- Storage engine unit tests were modified and cleanly pass
- Development environment: Ubuntu 13.10 with gcc-4.8.1