Remove unnecessary Item() constructor

Remove one (of the 5!) Item constructors - it was identical to another
apart from taking a std::string& instead of (char*, length) for the
key.

Change-Id: I3b123ee0c380e536bab6b8de8814db9e160db709
Reviewed-on: http://review.couchbase.org/42622
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>