Remove unused fields from `activeVbucket`.

Change-Id: Id3a680e5a83fee816b950fb283c074f278214491
Reviewed-on: http://review.couchbase.org/40867
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>