Normalize SwDBManager::Merge{MailFiles,Documents}

There is still a difference, if run mail merge throught one or
the other function (MM wizard or UNO).

Change-Id: Ia372bd3edea4c8dfb58e045fc8fed2efcb2a4fde
Reviewed-on: https://gerrit.libreoffice.org/10971
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>