docs: remove and ignore doc/doxygen_sqlite3.db

Some newer versions of doxygen are generating this file now, and if
we don't clean it up distcheck will fail.

Known to affect doxygen 1.8.8 from debian jessie.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>
Tested-by: Jon A. Cruz <jonc@osg.samsung.com>