removed excessive TitleField key from discount items and affiliate plan items configuration

Preparations - make api tab working

1. In the affiliate_plans_config.php uncomment 'api' in the 'SubItems' array.
2. In the affiliate_plans_config.php uncomment 'items' in the 'EditTabPresets' array.
3. Fix affilite_plans_items.tpl - add pagination_prefix="api" parameter to the RenderElement name="combined_header" tag.

Testing di unit changes

1. login to the adm. console
2. go to the E-commerce → Discounts & Certificates section
3. use adm.console interface in the current section to create new discount record, with some sub-records on the Items tab
4. confirm that records are created on the Items tab
5. go to the Logs & Reports → System Log section
6. confirm that no record with warning 'Field "Name" is not defined in "di" unit config' appeared in the System Log
7. confirm that no record with notice 'Undefined index: Name' appeared in the System Log

Testing api unit changes

1. login to the adm. console
2. go to the E-commerce → Affiliates section go to 'Affiliate Plans' tab
3. use adm.console interface in the current section to create new affiliate plan record, with some sub-records on the Items tab
4. confirm that records are created on the Items tab
5. go to the Logs & Reports → System Log section
6. confirm that no record with warning 'Field "Name" is not defined in "api" unit config' appeared in the System Log
7. confirm that no record with notice 'Undefined index: Name' appeared in the System Log