backends/brass/: Track doccount in the database stats.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18219 6b2f1b92-a4f3-0310-943f-de3cbe41748b
backends/brass/brass_record.cc,backends/brass/brass_record.h: Remove
BrassRecordTable::get_doccount() as it is no longer used.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18220 6b2f1b92-a4f3-0310-943f-de3cbe41748b