fix(user-center): remove rebate detail title setting

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Settings/PaymentRuntime.php

@@ -287,7 +287,6 @@ class PaymentRuntime {
             ],
             'text'               => [
                 'desc'          => sanitize_textarea_field((string) ($settings['pay_rebate_text_desc'] ?? '')),
-                'details_title' => sanitize_text_field((string) ($settings['pay_rebate_text_details_title'] ?? '')),
                 'details'       => sanitize_textarea_field((string) ($settings['pay_rebate_text_details'] ?? '')),
                 'discount'      => sanitize_text_field((string) ($settings['pay_rebate_text_discount'] ?? '')),
             ],