backends/chert/chert_check.cc: After we successfully regenerate
baseA, remove any empty baseB file to prevent it causing problems.
Tracked down with help from Phil Hands.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18328 6b2f1b92-a4f3-0310-943f-de3cbe41748b