SQL delete needs firmware check for older Kobo firmare

Kobo driver: Fix a regression when deleting empty shelves on Kobo devices with older firmware.
Fixes #1192441 [Private bug](https://bugs.launchpad.net/calibre/+bug/1192441)

As reported here,
http://www.mobileread.com/forums/showthread.php?t=214760, if the Kobo
device is using firmware before 2.5.0, it doesn't have the Activity
table. The delete from this table when maintaining shelves needs a
version check around it.