fix regression in eleveldb fixed indexes flag

introduced by b50d1df73a. the check assumed that is_empty
would return false when the fixed indexes key was written.
Since the change to is_empty accounts for this key and ignores it
lets just call the old version (eleveldb:is_empty) directly.
see comment in patch for more.
riak_kv_status:fixed_index_status/0

returns status of fixed indexes for the node. meant for use in
riaknostic