Bug 19646: fix value_builder marc21_linking_section template

Bug 19646: fix value_builder marc21_linking_section template

When using marc21_linking_section cataloging plugin, you get the error :
Template process failed: undef error - The method selected is not
covered by tests! at /home/koha/src/C4/Templates.pm line 122.

It commes from [% IF ( itypeloo.selected ) %], selected is not defined
in itypeloo.

This patch removes this condition because all item types are fetched
from database, there is not one selected. It sets "All item types" as
selected. Also correctes two errors in HTML : ending form and div,
"<a_class=" instead of "<a class="

Test plan :
1) Use a MARC21 database
2) Define marc21_linking_section cataloging plugin in framework, on
   773$t for example
3) Create a new biblio record
4) Click on icon in 773$t
5) Check plugin works fine

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>