WebSearch: better detection of deleted records

• webcoll now uses MARC tag 980 to detect deleted records instead of using the `collection' index, because this latter one was emptied in case of deleted records by bibindex, leading to a kind of Catch-22 situation between bibindex emptying the index and webcoll relying on it. Using physical MARC tag instead of the logical collection index fixes the problem. But beware if we would like to remove bibxxx tables one day. (closes #358)