Bug 19855: Remove $type from the alerts

Bug 19855: Remove $type from the alerts

It looks like this feature has never been finished. It has been
developed with more flexibility in mind, but only 'issue' is used for
this parameter. Apparently it could have been 'virtual', for virtual shelves.

Let remove this parameter and clean the code a bit.
TODO: Remove the DB column

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>