Bug 19563: Generation of sort_fields uses incorrect condition

Bug 19563: Generation of sort_fields uses incorrect condition

Fix incorrect condition for if clause for generating <field>__sort mappings
for Elasticsearch. Also remove redundant check for same condition when
generating fixer rules.

Test plan:
1. Inspect current mappings for example by viewing: http://<elasticsearch_host>:9200/koha_<koha_instance_name>_biblios/_mapping.
2. If using the default configuraion only "author" has a sort field (author__sort).
4. Appy patch.
5. Reindex using rebuild_elastic_search.pl.
6. All fields except those with sort sort set to "0" should now have sort fields, which in the default configuration is all but "author".

Signed-off-by: David Bourgault <david.bourgault@inlibro.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>