Migrate aggregator_category_selector variable to D8 properly

Follow-up to #2293419: Add config schema test to all configuration test in migration, fix bugs

Problem/Motivation

#2293419: Add config schema test to all configuration test in migration, fix bugs found that the migration system coverts the variable aggregator_category_selector to config in D8 however D8 doesn't have this variable by default.

Proposed resolution

D7 & D6 we are using this field value in aggregator_categorize_items (/aggregator => aggregator_page_last => _aggregator_page_list => aggregator_categorize_items), but doesn't exist in core/modules/aggregator/src/Form/SettingsForm.php form and not used anywhere.

Remaining tasks

Patch.

User interface changes

N/A

API changes

N/A