Bug 30667: Holds reminder cronjob (holds_reminder.pl) never uses default letter template

Bug 30667: Holds reminder cronjob (holds_reminder.pl) never uses default letter template

Bug 28514 changed the way holds_reminder.pl searches for templates, using a direct search for letters,
but should be using find_effective_template instead. Now, if a branch specific template does not exist,
it will skip that branch.

Test Plan:
1) Ensure you only have the default HOLD_REMINDER template
2) Become the koha user using koha-shell
3) Run misc/cronjobs/holds/holds_reminder.pl --days 7 -v
4) Note that the script skips every branch
5) Apply this patch
6) Run the command in step 3 again
7) Note the script doesn't skip over any branches

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>