Bug 9016: (follow-up) make GetLetters return all letters

Bug 9016: (follow-up) make GetLetters return all letters

GetLetters only returns letters with a mtt = email. It should return all
letter codes in the DB.

The message_transport_type parameter is never used.

To reproduce the issue:
Create a notice with a sms template and no email template.
Go on the overdue rules configucation page.
The notice does not appear in the notice list.

Signed-off-by: Olli-Antti Kivilahti <kivilahtio@ProBook6570b>
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Testing report:
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Testing this subroutine from a test stub. Calling the method without arguments
and with argument 'circulation' and 'circulat'.
Works as supposed to.
Related Bug 11931 discovered but not within the scope of this featureset.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>