Bug 18833: Plugin unimarc_field_210c pagination error

Bug 18833: Plugin unimarc_field_210c pagination error

In plugin unimarc_field_210c, the editors search results have pagination.
First page works but not other pages.

The main problem whas mainly because $startfrom var was missused. It is defined in C4::Output::pagination_bar as page number.
This patch corrects by using a $offset var.
Also removes dead code, obsoleted by using C4::Output::pagination_bar.
Also changes hardcoded results per page from 19 to 20.

Test plan :
- Use UNIMARC catalogue
- Define in framework value builder unimarc_field_210c on 210$c
- Create autority type EDITORS with heading 200$b
- Create 22 authorities of that type
- Index those new authorities
- Create a new biblio record
- Click on 210$c plugin
- Click on "Search"
- You see 20 results
- Click on page 2
=> Without patch you get empty table
=> With patch you get 2 results
- Check that you see "Results 21 to 22 of 22"
- Delete 2 autorities and retest

Works as intended.

Signed-off-by: Simon Pouchol <simon.pouchol@biblibre.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>