Bug 26985: Remove code for "Upcoming events" from codebase as not implemented

Bug 26985: Remove code for "Upcoming events" from codebase as not implemented

This patch removes references to an unimplemented feature for "Upcoming
events" notices. As far as I can tell the templates are the only files
affected.

To test, apply the patch and verify that no instances of "Upcoming
events" or "Upcoming_events" are found in the codebase.

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