Bug 9050: Use safer adelete when deleting records from Zebra index

Previously we used the "delete" command in zebraidx, which fails when
you try to delete a record that doesn't exist in the index. By changing
to the "adelete" command, we can reduce the likelihood of a failed
delete causing ghost records. A symptom of this problem is the warning
message occasionally encountered when indexing from the zebraqueue,
"[warn] cannot delete record above (seems new)."

To test:
1) Add a recordDelete action for a record that does not exist to
   zebraqueue in MySQL:
   INSERT INTO zebraqueue (biblio_auth_number, operation, server) \
       VALUES (999999999, 'recordDelete', 'biblioserver');
2) Run `rebuild_zebra.pl -b -z -v [-x]`.
3) Note that you do not get the message "[warn] cannot delete record
   above (seems new)".

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>